Something happens between 15:49 and 15:50. He either gets hit in the side of the left knee and/or leg, possibly ankle. Slowed down, the force of the hit seems to throw both legs out of further harm's way and he lands in a way where it does not appear to be abnormally twisted with no additional contact to the area. He immediately gets up planting on his right foot and then a second later, when he puts down his left foot, he stands there awkwardly for a moment as if hesitant to put weight on his lower left leg.  I've seen way worse (like Marv at ND looked much worse than this) but you can never tell with these things.