Won't necessarily have a 'reason'.
People react differently to different situations and it's all to easy to sit at a keyboard, analyse and give opinions.
Fortunately few of us have been in a similar situation to the rider in this case and none in exactly the same situation.
Very true, and sadly for me, I was put in a similar situation in my teen years whilst riding a motorcycle. I hit a pedestrian and pretty much broke every bone in her body, and in trying to avoid her, hit two cars that were coming in the opposite direction. I ended up on a pavement somehow having stayed on the bike. My first split second reaction was to run, not least because of the crowds of people that suddenly seemed to appear. I didn't and instead just dropped the motorcycle and ran to help.
I truly don't know if I would have handled things differently, had it not been for the fact that my best friend at the time, had been killed by a hit and run driver just months previously. It was the recollection of that, and the impact of what followed, that rooted me to the ground. I have had to deal with the fact that I was involved in an accident that basically killed someone, and had I ran, I know that I couldn't have dealt with it. A clear conscience is bad enough.
I was found not to be guilty of any offence as the pedestrian had been drinking and stepped out from nowhere. It was still over five years of hell for me and I'd dearly love to be able to turn back the clock, or even swap places with the pedestrian. I have pretty much spent my whole life since, stupidly punishing myself, and it has has only been the last few that I came to the conclusion of why?