This has happened to me a few times too. First time was a full body shake second time was just my head. I have read that Out of body experiences are a sign of epilepsy and we all know that seizures are as well so i can't help but think there is some kind of connect even if it is purely biological. Maybe something in the brain is short circuiting. Another theory could be that you are not relaxed enough and the mechanism that shuts off your muscles when you fall asleep isn't completely turned off and you are kind of unconsciously acting out the vibrations.

What i can say is that both times this happened to me i was not relaxed enough. This hasn't happened to me since i started relaxing into the vibrations. Once you start relaxing into the vibrations they go away.It can be a little tough because the vibes are intense but next time you get the vibes try saying to yourself something like " it's ok, just realx into it it will pass". Feel yourself letting go when the vibes start and attempt an oob exit technique.

When the vibes are full force the best technique that has worked for me so far has been to look up with my eyes to the brow center.

Best