The stills and footage make it quite clear that she had a bag over her arm and papers in one hand and her phone in the other, he would have as be had time to observe this before he assaulted her. She was quite clearly intent on walking past him so his defense of an instinctive reaction is bollocks. We can see her instinctive reaction as she cowers away from him.
Whilst the protest was intended to be disruptive, that's the point of it, it does not excuse the level of violence Mark Field used. Blocking her path and asking her to leave or waiting for security to escort the protester out would have been reasonable. His response was driven by anger not fear, it almost looks like he's trying to reign himself in from actually going further.
He needs to go and the media needs to stop being complicit in minimising male violence.