It really looks like more of a reflex he hits her. I know that can happen. My sophomore year in hs, this bitch went off on me when I was talking to this other girl she was talking to. I didn't interrupt her, so I don't know what her problem was. I also lived not too far from her, and we knew each other pretty well, though we were only acquaintances. We yelled at each other and shit and then she attacked me not once, but twice. Each time I held her down and told her to stop, would let her back up and she'd attack me again. The 3rd time, she stood up and kicked me in the balls. Without any thought and as much as suprise to me as anyone, I popped her in the face and gave her a bloody nose. A senior came to 'visit' me later in English class only to back off when he saw it was me; knowing I wasn't the type.
In summary, shit happens.