According to the fan who sent the video to Deadspin:
Fight started in the stands with two girls fighting. And moved out to where I shot the video. Beer was thrown and clothing over the balcony. Security went after the guy. Didn't see what happened after that.
I don't know. To me, anything less than the girl vice-gripping his balls, using an actual vice, probably doesn't warrant punching her in the face. Also, what took security so long here and why didn't everyone jump this guy once he hit her?
H/T Big Lead