Exercising in Sleep

We have been experiencing some leg dropping in this past week.  Koekje likes to drop his legs so hard that it shakes the crib and we can hear it downstairs.  :)  He also does this throughout the day.  He lifts them up and slams them into the ground.  Here is a video of him doing this in his sleep.

I wonder why he is tending to wake up shortly after we put him to bed lately.