Why is this guy blocking the rear wheel?
No reason at all.

NWD10 is a classic old school movie. You don't see purposeless skidding like that in today's  MTB movies nor do you see it in real life MTB riding. Kirt Voreis (the guy in that scene - legendary rider) is probably doing this for no reason, just for fun, or at least what used to be fun in these old times.