If the modern manifestation of the unknown tongue movement is not Scriptural (not from God), then what is this present phenomenon attributed to and how can it be explained?
1. Psychological cause
The 1828 Webster dictionary defines “psychological,” “as the study of the soul of man.”  A large amount of unknown tongue speaking is sourced in the psychological realm of the human soul. Paul told the carnal Corinthians, “…in the spirit he speaketh mysteries”  (1Cor.14:2), meaning that the incoherent, chanting, unknown tongue of the Corinthians was for the most part produced within themselves and not by the Holy Spirit (spirit is with small “s”).
Human beings are very open to suggestions. Let’s take a very simple example, the yawn. In a room full of people someone yawns, and before long, so does nearly everyone else.  Another example is someone who is experiencing a strong emotion joins a group. It has an immediate effect on everyone present.  The power of suggestion is found in music. Music, to a high degree, affects our emotions. How many times have you found yourself whistling or humming a tune even though you dislike it?  This sometimes happens involuntary by or unconsciously. When the false gift, such as unknown tongue speaking, occurs in a church, it invariably becomes a spiritual status symbol and the power of suggestion soon spreads it throughout the church.
In the modern tongue movement today, its leaders or practitioners use methods or techniques of suggestion in order to induce people to have an unknown tongue experience. However, the leaders are usually unaware of this. People are told that speaking in an unknown tongue gives them special favor with the Holy Spirit and power over demonic strongholds.  This creates in people a desire to have the experience manifested in their lives.  The person who desires to speak in an unknown tongue is encouraged to switch off his mind. They are exhorted “to go with the flow of the spirit.”  Do nothing, don’t think or try to reason what is happening. Experience first, analyze later. Launch into the routine and surrender to what you feel, and whatever happens, do not resist.
The usual method is that one or two leaders lay hands on the person and begin praying out loud in an unknown tongue (a repetitious gibberish).  When the human mind is cleared from all objectivity it begins to experience differing degrees of what is called: “altered states of consciousness.”  In this state, one is completely absent of any discernment.  A major effect of an altered state of consciousness is that a person becomes highly prone to suggestion.  A person hearing the continual utterances of those praying over them in a loud, emotionally charged, repetitious babble, records in his unconscious mind the noises being heard.  Since the objective mind is silenced, ones suggestibility is influenced by the outside stimuli and automatically the person begins to repeat with his unconscious mind what he is hearing, sometimes even without any exercise of control.  Suggestibility induced by the practitioners, creates a desire so strong in the soul of an individuals that it bypasses the conscious control of speech in the brain. The subconscious (subjective) mind takes over the tongue and produces whatever is expected or strongly desired.
Psychologists have known this to occur with people who have no strong religious convictions at all. It is a known psychological phenomenon. Some psychologists and hypnotists have called this state “motor automatism.”
Webster Collegiate Dictionary, definesautomatism as:
The quality or state of being automatic; an automatic action, the power or fact of moving or functioning without conscious control either independently of external stimuli or under the influence of external stimuli.
This psychological phenomenon is purely soulish or carnal in nature.  It is completely void of the Holy Spirit. Yes, it may be an honest experience.  It may even feel good and produce a sense of spiritual authority. Yet this is where people get confused. Many deceived and ignorant Christians consider an inner "feeling" to always be God’s voice and then act according to what that "voice" says. In other words, their actions are determined by feelings and desires instead of reason enlightened by the Word of God. They judge their experience by some preconceived notion, presupposition, or pre-set attitude of mind. They explain it in terms of what they want it to be. They believe what THEY want to believe about it and then they use their experience to prove their claims. That does not prove anything, of course. The facts of the experience may be undeniable, but the explanation of why it occurred must only come from Scripture.  
No matter how true an experience may be, the Scriptures must be the Christian’s final authority. That is the purpose of Scripture. It is given to us so that we might be illuminated and enlightened in the understanding of our mind by the Holy Spirit. As we read and search to know the truth, we are able to distinguish between spiritual and carnal (1Cor.2:15-16). There is no Scriptural justification for deliberately seeking for altered states of consciousness. Quite the contrary, the Scripture contains numerous injunctions to be “SOBER,” “ALERT,” and “WATCHFUL,” particularly against being deceived (Mat.24:42; Lk.21:8; Eph.5:6; 1Pet.1:13; 4:7; 5:8). The Apostle Paul told the Corinthian tongue speakers that they were carnal, not spiritual (1Cor.3:1,3,4). Paul tells us that “the carnal mind is enmity against God” (Rom8:7). 
Because this is a psychological phenomenon, some people, who by their constitutional make-up are unable to respond, face a very discouraging situation. There are some wonderful Christians earnestly desirous of having everything that God wants them to have and wanting to go on with God, who are deceived into thinking that tongue speaking is a spiritual mark of divine favor. They struggle and strive to receive this gift but never get it, and eventually fall into despair. 
Modern-day tongue speakers manifest their carnal nature in their reaction to those who do not speak in an unknown tongue.  They do this by looking down at other brothers and sisters in Christ with arrogance and pseudo spiritual superiority.  I have personally encountered these self-righteous attitudes over and over again among the charismatic circles. The “we have this gift or the power and you don’t” attitude is anything but spiritual! These people lack the true manifestation of CHARITY, by which Paul said, all gifts of the Spirit were to be exercised and revealed (1Cor.13:1-7).
2. Artificial
The second possible source of an unknown tongue (an incoherent babble) is artificial or manufactured.  What does this mean? The person desires to have a genuine spiritual experience but actually does not have it.  Therefore, the person imitates the sounds he hears from others in the church and at other times he is directly told to repeat the sounds he hears. They are told things such as, “Now pray audibly but don’t speak English.” Or, “Start to speak syllables—just let it flow.” When he does, he is told he’s speaking in an unknown tongue! I have attended meetings where the leaders shout at and command gullible people to speak in an unknown tongue.  This is one of the main indicators that reveal modern tongue speaking to be fraud.  The majority of charismatics are taught how to speak with an unknown tongue by their peers.
“Speaking in tongues is a venture of faith. You lay aside any language you have ever learned, then lift up your voice and speak out. The risk is that you will say nothing more than bla-bla-bla. But when you take this step of simple faith, you discover that God indeed keeps His side of the bargain and begins to shape the sound which you continue to give Him into a language of prayer and praise.” 
Larry Christenson,  Speaking in Tongues
Do we encounter anyone in the New Testament being instructed on how to pray in tongues? NO, the exact opposite is the case. Those who spoke in tongues in the book of Acts, for example, never asked what to do, and were never told to do or say anything. In the biblical accounts people spoke in tongues spontaneously. In Acts 2:4, 10:46 and 19:6, those who spoke in tongues did so with no prompting or preparation. In fact, in each case, those who spoke in tongues, prior to receiving the gift, did not know such a thing as tongues even existed! Thus, not only is the modern tongue irrational nonsense opposed to the real foreign languages spoken in the New Testament, but also the manner in which they receive tongues is completely different from that in the biblical record.
"Instead of accepting a person on the basis of what he believes which has always been the Christian way, it’s do you have these external manifestations?”
Francis  Schaeffer, Christian Apologist
Those, who speak in an unknown tongue artificially, usually do it in order to be accepted by the church.  It is the acceptable thing to do (peer pressure).  So, tongue speaking becomes a learned behavior, motivated by the desire to have a status symbol of spiritual achievement in the church.  The tongue experience is stimulated by the desire to receive praise and recognition from others. Though tongue speaking can be classified as artificially manufactured, the person who does so is deceived.  Deceived by his own desires. The fallen, carnal nature works mainly through desires, and since the fall, man has listened to his desires more than his reason.  Christians, who follow after their desires instead of the authority of reason enlightened by God’s Word, will be deceived by their experience, feelings, passions, and impulses.
3. Demonic
“A major problem in the Charismatic movement: a desire for power and at the same time a denigration of biblical doctrine, with the resultant loss of the very guidelines which would protect one from occult involvement.”                                                      
Dave Hunt, Occult Invasion
Usually, when we hear someone say that we need to distinguish between good and evil, we think that is very easy. Evil (satanic) looks evil and good (from God) looks good, and anyone can tell the difference. But it is not so simple. The whole danger about the work of Satan is his ability to deceive (Rev.12:9), his ability to act with subtlety (Gen.3:1; 2Cor.11:3), and to set snares and traps (Eph.4:14;6:11). Anyone who knows anything about the use of snares and traps knows that no one sets a snare that looks like a snare, nor is a trap laid out in the open where everyone can see it. It is DISGUISED or hidden so it will not be openly recognized. Therefore, a so-called supernatural experience that is exciting is not always sourced in God (Ex.7:10,11,22;8:7).  
2 Corinthians 11:14
And no marvel; for Satan himself is transformed into an angel of light.
This is the chief characteristic of the devil. He can appear as an angel of light and deceive (lead astray) people by “power and signs and lying wonders” (2Thes.2:9) and “…wrought miracles…with which he deceived them (Rev.19:20). Therefore, there is a VITAL need for distinguishing the spirits (1Jn.4:1), for recognizing true from false, and knowing the difference between good and evil. The entire New Testament continually admonishes us to this end (Jn.8:31-32; Acts 17:11).
The Scripture must always be the test of what is good or evil, never a person’s experience.  I would like to emphasize that. It is no good for someone to say, "This happened to me, therefore, I know it must be of the Lord," or, "A friend of mine went through such and such an experience. It was so wonderful that he tells me that the result in his life has been a great blessing, therefore it must be of the Lord." Now, if you judge things on that basis you are certain to fall into the trap of Satan. That is precisely what he uses to deceive. He is always trying to get us to transfer the basis of our action from what we know (objective truth) to what we feel (a subjective experience).
Satan was doing that with the Lord Jesus Christ in the wilderness temptation when he said, "If you are the Son of God, change these stones into bread," (Matt 4:3). That is, "Do not any longer continue to walk by faith in God’s Word, by which man is to live, but rather show something, demonstrate something, experience something supernatural." This is the continual effort of Satan- to transfer us from faith in God’s Holy Word (what we know) to experience (what we feel and desire). Jesus did not respond to Satan’s temptation with a foolish cliché like, “I bind you, Devil.”  Instead, He said,  “ It is written, Man shall not live by bread alone, but by every word that proceedeth out of the mouth of God (Mat.4:4).
Consequently, never take anyone’s experience as absolute truth (Col.2:18), even though it may be valid. What happens is one thing; the explanation of it is quite another.  Christians are responsible to live by what is WRITTEN, not by what happens.   “It is written…” Christians will never be deceived if they humbly conform to God’s will found in His infallible Word instead conforming to their own will found in their self-centered emotional experiences.
“People have assumed, because the name of Christ has been used in a meeting, that all that happens in it must be truly Christian, and is, therefore a guarantee of the soundness of all that is taught. For them the results guarantee everything…they have failed to take heed to our Lord’s warning in Matthew 7:21-23.”
 D. Martyn Lloyd-Jones, Studies in the Sermon on the Mount
The modern, mystical, unknown tongue practiced today among Charismatic/Pentecostal circles is typified by an incoherent, almost hysterical repetition of syllables over and over again. Sometimes it is the same syllable repeated endlessly in a continual jabbering. This insane confusion is simply not of God. It is a perversion of true biblical tongues (i.e. national foreign languages). It has entered the Church from satanic, eastern religions and through private interpretation of 1Corinthians 12-14.  Private interpretation means to isolate certain passages of Scripture from their immediate context (2Pet.1:20-21).  When this is performed, one can make the Bible say anything.
Satanic deception in the last days will not only take place outside the Church, in the world, but also inside the Church (Acts 20:29-31; Eph.4:14; 1Tim.4:1; 2Pet.2; Jude). The only worldwide revival in the last days will be a false revival. We live in a desire-indulged, feeling-oriented, experience-centered time of history where many of God’s own people “…will not endure sound doctrine; but after their own lusts…shall turn away their ears from truth…” (2Tim.4:3-4).
The Church, in the last days, will be a biblically ignorant Church lost in the delusion of materialism, secular psychology, a passive comfortablity, and overcome by worldly entertainment (music,TV, and movies). Sound doctrine taught from the Bible will be called boring, not anointed, and only “head knowledge.” Truth will only be with the minority. Crowds of Christians will flock to churches seeking experiences, which make them “feel” close to God. They will go to churches where the pastor tells them only what they desire to hear. Experience will be the measuring rod of truth in these counterfeit, emotional movements in the last days.  If it feels good it must be God!
“I’d rather be in a church where the devil and the flesh are manifesting than in a church where nothing is happening because the people are so afraid to manifest anything…and if the devil manifests, dont worry. Rejoice because at least something is happening.” 
Rodney Howard Browne, Charismatic Revivalist

"Just be joyful…God is not serious…this world cannot fit with a theological god…the goal is to create a new man, one who is happily mindless.”
Bhagwan Shree Rajneesh, Kundalini Yoga Guru
Nevertheless, Christians, who seek after spiritual experiences without any Scriptural support, will be led into demonic traps. Any part of the mind, which is not filled with doctrine (how to think with God) is open for satanic deception. What if the Christian is sincere, doesn’t God look at his heart?  A person can be sincere and be sincerely wrong and deceived.  Remember, Peter was perfectly sincere and honest when He rebuked the Lord Jesus for talking about His crucifixion. Peter (a believer) was entirely desirous of doing the right thing and he thought he was doing so. Yet, Jesus said that he was being led of Satan (Mat.16:22-23).If a Christian is sincere concerning his high calling, he will seek to obey Paul’s clear command:
2 Timothy 2:15-16
15 Study to show thyself approved unto God, a workman that needeth not to be ashamed, rightly dividing the word of truth. 16 But SHUN profane and VAIN BABBLINGS: for they will increase unto more ungodliness. 
Remember, in order to obey this portion of Scripture correctly long hours and hard work is involved. Scripture must be compared with Scripture. Yet many lazy and slothful Christians cannot be bothered with doing that.  They would rather “desire to make a fair show in the flesh” (Gal.6:12), praying in pagan mantras (an unknown tongue), and call it the baptism of the Holy Spirit (Mat.6:7). Paul told us that we should SHUN PROFANE AND VAIN BABBLINGS! The word “babble,” according to Websters 1828 Dictionary, means: “to utter sounds frequently, incessantly, or indistinctly; as a babbling echo; a babbling stream; senseless irrational prattle.” Paul said it “WILL INCREASE UNTO MORE UNGODLINESS.”  Evil spirits can easily find a place in the believer’s emotions, especially when he is told to suspend all intellectual activity (discernment) and yield his will to an invisible intelligence, which they believe to be the Holy Spirit.
Speaking with an unknown tongue bypasses intelligence. Beware of any action of the spirit that ignores the God-given faculty of thinking.”
Greek Scholar Spiros Zodhiates, Why Speak with Tongues
The very fact that unknown tongue speaking has its origins in eastern religions should be enough to tell Christians with any common sense, that it is a forbidden tree.  Pagan religions and eastern mysticism such as Buddhism, Hinduism, tribal voodooism, spiritualism, and even Islam have all reported manifestations of ecstatic babble called an “unknown tongue.”  Ecstatic utterances were used in the Greek oracles and in the ancient mystery religions. Spiritualists use the tongue-speaking phenomenon in their claim to communicate with the dead.  Even today, in Rhodesia is a heretical sect known as the Apostles’ Church, which combines certain elements of Christianity with the traditional pagan practices. Tongue speaking is a part of their ceremony. Linguistically, there is no distinction between the modern “unknown tongue” speaking among charismatics and those manifested in demonic by inspired religions and cults.  The fact that the tongue-speaking experience is found in false and contradictory religions would obviously indicate that its source is not from God but a human emotional experience confused by the devil as a divine encounter.
The mantra method in eastern religions today is the most commonly employed technique to silence the intellect.  These absurd religions consider the mind to be an enemy of the spirit.  Therefore, it must be set aside by techniques that cause it to cease functioning.  Fastings, lotus positions, long periods of silence, and repetitive mantras are just a few of the techniques employed to stifle objective thinking.  By repeating or chanting a word or syllable over and over, the neurosensory faculties of the body become fatigued and shut down.  It causes the one who meditates to lose touch with the objective meaning of the word or syllables and then enter into altered states of consciousness. The altered states of consciousness is the merging of ones consciousness with the Universal Mind.  This realization is the goal of Eastern mysticism.
Those who practice mind numbing mantras often report experiences of peace and   feeling of joy, leaving ones body (out of the body experiences), and indescribable emotional bliss.  These are the same experiences Christian’s testify to have had when they pray in an unknown tongue. 
“Slowly I began to lose my own identity, too… This is quite an experience, losing consciousness of self. And I was helped by gaining, at the same time, the awareness that another presence was in the room…the light blazed through my closed lids, blinding, dizzy, and fearful.  I was afraid of this approaching contact….”
John Sherrill, They Speak With Other Tongues
Experiences like these become habit forming to the flesh (just as a psychedelic drug, such as heroin or LSD). This is one of the main reasons why it’s so hard for a tongue speaker to stop practicing it.  They become very defensive and reactionary toward those who oppose them. No matter how many Scriptures you show them concerning the doctrine they reject it because their reason is blinded by the desire of having a subjective experience. These blind Christians forsake enlightened reason and are not interested in the “facts.”
“Those who have spoken in a tongue under demon power are subject to nervousness. They become impatient and irritable. At the same time, they boast of great spiritual power. They cannot submit to opposition or correction. They are intolerant and very dogmatic. They become extremely touchy when anything is done or said which disapproves or corrects speaking in an unknown tongue.  ‘The wisdom that is from above is …easy to be intreated’."
Dr. Ruark, Falsities of Modern Tongues
For those carnal Christian’s, who insist upon having a so-called gift of the Spirit and ignore the Word of God concerning its purpose and use, there remains the dreadful, possible reality of the unknown tongue being rooted in demonic power.
James 3:15-17
15 This wisdom descendeth not from above, but is earthly, sensual, DEVILISH.  16 For where envying and strife is, there is confusion and every evil work.  17 But the wisdom that is from above is first pure, then peaceable, gentle, and easy to be intreated, full of mercy and good fruits, without partiality, and without hypocrisy. 
There are only two sources. It is one or the other. Anything that arises out of the carnal mind, the flesh, the human, Adamic, fallen nature is devilish. It oftentimes has enough novelty about it to capture the imagination of people and fascinate them, even help them in some temporary fashion. But if they shall eventually end up in very serious trouble, especially at the future Judgment Seat of Christ, which is the future judgment for all Christians regarding their practical walk on this earth (Mat.7:21-23; Rom.14:10-12; 1Cor.3:13-15; 2Cor.5:10; Heb.10:26-31). 
At this future Judgment, the Lord will not judge according to one’s heart attitude, but rather, in light of how a Christian acted by faith according to the knowledge of His truth found in the written, objective Word of God (Mat.7:24-27; Col.1:9-10; 2Pet.1:3-11; Heb.10:38-39).  The written Bible, “which was once delivered unto the saints” (Jude 3), shall be the only means by which the Lord Jesus Christ judges His people in that day (Jn.12:48; Heb.4:12-13; Rev. 1:16; 2:16).  Doctrine does matter! (1Tim.4:16). Christians will find themselves in one of the two positions after that day: approval or disapproval (1Cor.9:24-27; Mat.24:45-51; 25:1-13). Both of them have consequences (positive and negative) which extend over a period of 1,000 years during the millennial kingdom of Christ on the earth (Mat.19:27-30; 25:14-30; Lk.19:12-27). 
Isaiah 8:20
To the law and to the testimony: if they speak not according to this word, it is because there is no light in them.