I did watch the video a few times, but it's hard to really tell. I would like to hear what he has to say, but the thing is, if he didn't know the woman then there's surely no motive other than whatever happened in the split second they met. There's a moment where her leg flies out in front of where he's about to run to -- I'm being deliberately ambiguous in that sentence because I think it's a little unclear whether she stuck it out deliberately or only as she was already falling, but if he thought she was trying to trip him up then he might petulantly have pushed her out of the way without thinking what he was pushing her in to.
He's been arrested and tracked down, and rightly so, but I do want to hear his own excuse before passing judgement. It was, at the very least, monumentally stupid.