I have been having the strangest experience since I was 19. I am sound asleep, probably during REM, when suddenly I become aware there's a presence just outside my range of sight. So I try to open my eyes and turn my head (body, whatever movement is necessary), but I can't. At this point I begin to panic and struggle inside my head trying to break the paralysis which eventually I do, but it's incredibly difficult. I don't move during these struggles and of course, there's never anyone or anything there lurking just outside my field of vision. By far the worst one was when I could see the 'presence' standing over me, and he was an executioner with a very large axe. Just as it started down towards me, I managed to break the paralysis, but in my panic to get away from him, I slammed into the wall - right where the corner is, and broke my nose. Hubby says I redefined the term 'face-plant'. :facepalm:
My therapist had one hell of a theory about it in our session this morning - basically, he thinks the child part (child/adult/parent) of my psyche is taking control and she does this while I'm asleep because it's easier for her. I have to admit, of all the theories and reasons I've heard over the years, his actually makes sense. The little girl inside me is pretty powerful. She holds her own, so it makes sense to me that she would take control while I'm asleep because SHE fears the monster is never far away and her hypervigilance is over the top sometimes.
So my answer to your question is, I don't know. I just know I'm seeing a lot of similarities on this thread that have me intrigued.
Oddly enough, until this morning I had no idea that sleep paralysis doesn't happen to everyone. I'd always thought it was 'normal', whatever that means.