Going vertical for headers puts you in danger of other players colliding with you. By raising your knees and legs you shield yourself a bit. Might not be the cleanest play but it's smart. They're considering making kids wear helmets at the high school level because of head injuries from mid-air collisions.

I don't dispute that he's an asshole, but some of his actions (like the Muller headbutt or the kicks to that first guy in the video) are out of frustration at the flopping and diving.