Looks like she's in his ear because he kept shoving into her, trying to force himself into that area disregarding her position. She got upset that he was pushing in, so she starts mouthing off, seemingly telling him that she's going to punch him in the face if he doesn't stop pushing into her. The guy sees her arm/hand positioned as if she's threatening to deliver a punch, so instead of apologizing and backing away, he instead decides to restrain her and talk smack back at her. In order to free herself (it looks like she's telling him to get his hands off her), it's possible that the knee was delivered at that moment, followed by the weak punch that's then countered with a disproportionate strike by the guy.
The woman definitely could have handled the situation differently, but it looks like the guy pushing into her, invading her personal space and grabbing her was what escalated the altercation to the point where she felt the need to knee and/or throw a punch. None of the actions by the woman necessitated that blow to the face.