So, before this goes down I noticed the bartender not wearing the jumper pouring a picture of mixed drinks, so clearly a pretty typical college binge drinking bar.
I also notice substantial competition to get the bartender's attention.
The chick who got punched establishes position to order from the bar.
Our hero pushes his way to the bar rail, pushing around her, crowding her, maybe trying to jump line a bit, maybe rubbing up against her a bit.
Chick takes offense and has comments.
He grabs her and restrains her arms when she begins gesticulating.
She throws an entitled ##### punch at him that would have been ineffectual had it landed flush. Typical drunk ##### move, presuming that her gender would protect her.
He throws a haymaker and connects.
His action was disproportionate, particularly given his initial attempts at line jumping to get to the bar.
My guess is that she has been an accident looking for a place to happen for some time.