To me it's as plain as day when you watch the video.
AG is gesturing wildly and shouting but continuing forward, at the moment CW passes her, she has stopped, turns towards CW, you see her raise her arm and extend her open hand out towards CW, you see her weight shift in her legs from her right to her left, at the moment of the push, you see her lean over and a there is a judder in her left trouser leg that makes it look like her hand came into contact with some resistance.. then at that exact moment, Celia veers wildly to the right and falls into the road.
On rewatching the video, it's quite obvious that AG pushes CW left arm forward which is what causes her to turn right so sharply.
the pics attached show the just before and the just after contact is made.. the issue with the proof is that the actual push itself is hidden behind AG body at the time because of the position of the camera, but you can see her arm extending before and see the position of her legs/body in the second photo and that her arm is returning from a position slightly behind her.
Plus the fact that she admitted having 'contact' with CW.