Watch it again. At 0:14, he's shoving his way to the bar from behind her, and something about how he does it upsets her. At 0:20 you see him grabbing her right arm (initiating the assault). She responds to his grabbing her by hitting him at 0:22, and he responds by socking her.
Hard to believe, isn't it, that people here are still trying to defend this thug/punk QB.