Why would I automatically assume these experiences weren't from God?
It is true that such experiences could be demonic in origin. They could also just be the power of suggestion. The best metric is to observe the effect of this experience on the person receiving them. Do they draw closer to God or do they shy away? Do they embrace their fellows in ever growing love or do they become embittered? I think context matters. Ultimately I am very hesitant to squash what may very well be the Holy Spirit at work unless given compelling reason to do so.
You asked a very good question.
What I will do is to argue from silence (somewhat), and point out what contexts such things DO manifest: The scriptures give no examples whatsoever of true believers convulsing, being "slain in the spirit" falling over from some dude waving his coat against them, sounding like animals, their bodies taken over by Holy Spirit to do things such as run around the building, gyrating around out of control, etc. The instances when we DO see those types of things manifest, it's, as you said, in the context of demonic possession. I cannot recall one instance of Holy Spirit animating someone beyond their own control. The Spirit leads us, guides us, speaks into us, but never taking over control of our bodies. People can and do give over control of their bodies to demons, who willingly take control, but that is not how the Lord operates.
Additionally, if we introduce into our own understanding any level of doubt about observed manifestations such as these, we then fail to properly deal with such in the lives of who we may or may not assume are our brothers and/or sisters.
Granted, some of these manifestations are a matter of mind control and power of suggestion from various circus acts that have learned the subtle art of manipulating the minds of those around them who are more susceptible to such. I saw a video yesterday of some guy in the mall selectively finding what looked like a girl more easily manipulated and prone to crowd participation and crowd pleasing, and he allegedly got her to forget her own name. The human mind being the vastly complex organ that it is, coupled with an even more complex spirit and soul within, we can't say for sure how those mechanisms function, but there are some who have learned to manipulate those who are willing to give themselves over to suggestions and the desire to "play along."
Your thoughts?
MM