when i was in high school i frequently experienced this - every afternoon nap i think. it was frightening. but then i researched about it and a good explanation for this is that our body is just trying to keep us stay asleep. hence, our bodies hormones are "paralyzing" our body in order that we still maintain asleep. it is just because we still lack sleep the moment we sleep paralyze.

you know it still occurs to me but now that i know i just relax and get back to sleep.