It is a fairly common event with people. Your body basicly does become paralyzed when you hit REM. What seems to happen is you wake up before your brain has turned everything back on.
This disorder also can lead to other fun and down right horrific things, such as being awake yet still in a dream state. Where you know you are awake, you see your room etc, yet your brain is also adding in dreams or hallucinations. So you could wake up then see some scary ass thing walk out the door.
More info here.
http://www.arts.uwaterloo.ca/~acheyne/S_P.html
My personal one I hate about sleeping is when you stay up a long time and are really tired. You lay down and you suddenly feel like you are falling or sinking fast. It will actually cause you to jolt. I think it feels alot like you believe you are dying, or slipping out of your body. This one happens to me a great deal.
The sleep paralysis is more rare for me, but then again I am already paralyzed.