probably a stress response from people who don't know how to be honest enough with themselves to express their pain and frustration in any way at all
>I have no mouth and I must scream
so they go into shock and have extremely weird panic attacks manifesting as nervous system glitches and an outpouring of all their negative emotions in the form of babbling things that they know society believes is wrong because it's some kind of release for them, and thus they appear possessed.
exorcisms happen and they kind of get back up all embarrassed and slink back away to their lives and try to forget about it. probably works because the chanting and praying and the general embarrassment of being in a room surrounded by people watching them spaz out is enough to overwhelm the senses enough to drown out the psychological and nervous system hiccups and panic attack and bring them back to a more stable state of consciousness.
it's usually the most lost and ignorant and forgotten generic plain boring normal and poor who get affected.