Boys will be boys.
UPDATE: Yes, he really is kicking his brother in the face, repeatedly, so hard he's actually airborne in this shot, while also biting him on the leg. It's amazing what they do to each other just playing, without any apparent effect. They can't attack me this way; even through two pairs of wool socks it tears skin off and I have to make them stop (they actually have to work pretty hard at not hurting me when they play, which is sort of ironic considering I outmass them by over 10:1). I guess that fur is better protection from claws and teeth than it looks.