Her voice is so mild when she rebukes him, that I doubt she is making an obscene gesture. I could easily imagine it being a forefinger raised as a non verbal request for him to halt. It's really impossible to tell.
Ok, I've had another watch/listen.
He steps right in front of her as if she wasn't there. She says "Please don't knock me off" He comes back with "Shut up, you <garble>**" I reckon her finger goes up then, in the classic American style. He chases her, knocks her down and abuses her.
I think it's a matter of opinion who started it. Personally, I'm on her side. But if I was going to give the finger to someone I thought was going to knock me down, I'd make sure I had a getaway route available.
** I can guess what the garbled word was.