It happens to me on occasion, and really I just want to be left alone in a dark, quiet place with no people around until it passes. I should note that by 'no people' I actually mean no people, even people that I really like and almost always want to be around me. It's like a strange little snap in the brain and is terribly unpleasant in general, and more than a little scary. After a little while it'll pass and I'll be perfectly okay, though usually very tired and ready to sleep. Actually, now that I'm thinking about it, it's about the only time I can reliably get a decent amount of sleep.
As to what causes it; on a chemical level I have no idea, though any overwhelming situation, usually with lots of noise and bright lights with a lot of things going on at once, can be the trigger for me.