In this particular case, according to journalists who viewed the exchange, it is certainly possible that Molitor was defending herself.
During what is apparently a contentious exchange of words, Molitor pushes Mixon away. We don't know why: there is no audio on the videotape. It is possible that she is the aggressor in pushing Mixon. It is also possible that he says something threatening to her and she pushes him away, defending herself. Mixon turns and lunges at her. Was it reasonable for her to feel his lunging was a physical threat? In other words, was she justified in slapping him to defend herself? The slap did not hit his face but rather his neck. He punches her hard enough that she drops to the floor and is unconscious and bleeding. He is 6'2, 216 lbs and extremely muscular and fit. She (if I have the right person) weighs more than 85 lbs less than he does and is 7 inches shorter. To the best of my knowledge, she is not an athlete.
The comments/tweets made by the journalists who were allowed to view the video tape is that the punch was really stunning, hard, shocking, etc.
Reading this, even if I remove gender, it is hard for me to say that the person who pushed/slapped at the other person was not justifiably trying to defend themselves. A lot would hinge on audio. There were lots of witnesses but I haven't found any testimony about what was said. The push/slapper could have been the aggressor or could have been defending themselves, as far as what we actually know. The person punching the other person seems, according to viewers of the video, to be acting far out of scale of what would be a reasonable response. Perhaps they are wrong. Perhaps she said she had a gun and would shoot him. That might be ample justification. But other wise, I cannot see how there was justification in the punch.
No-one's saying the proportion of the punch was justified. No-one.
But shoving someone, unless it's in self-defense or an accident, is both violent and an assault. Calling shoving 'not violent', calling a slap 'not violent' is doing violence to the English language.
Australia has recently seen a wave of one punch assaults that have resulted in brain damage and death. People need to stop shoving, slapping, and punching others. It doesn't matter if you're weaker than the other party.