When you get close to sleep paralysis, your body may send a strong signal to the mind telling it that it's uncomfortable, and to 'do something about it' so to speak. My guess is that this can be through the eyes as well, in which case you need to bear the discomfort and not move, until you enter sleep paralysis. However, if you're experiencing this while in SP and trying to fall asleep, I would suggest relaxing your eyes and allowing the rapid movement to happen, because it sounds like you're entering REM sleep.

ShockWave.