FYI a lot of those gifs on floating around are pretty misleading, and make it look like he grabbed her unprovoked.
You can see at the beginning as she gets to the bar she looks back at him for a good bit, and then continues to try to box him out, even extending her left leg so he couldn't get through. The second she turns around she has her right hand in a fist, well before he grabs her. OF COURSE he shouldn't have hit her, but it seemed like a heat of the moment thing more then some thing premeditated or malicious. It's not like he kept wailing on her. Honestly, this whole situation seems to be two drunk idiots behaving like just that.