I suspect the same thing, that he never saw what happened, or knew it was not his fault and just kept going.
I once saw an old man try to kill himself on route 78.
He came out of the grass median, perpendicular to traffic.
I saw it coming, and I avoided the hit. In my mirror, I saw a cloud of dust when the guy was hit broadside by a truck that was behind me. A big explosion. I think he was intentionally trying to kill himself.
Who knows with the old folks. They sometimes get confused. My mom is 84 and still drives. I know, that at times she is dangerous. She is beginning to realize it too. But that doesn't stop her. She had an accident about 4 years ago. It wasn't her fault at all. She was stopped, and left a gap between her and the car in front so a truck could get out from a side street. Turns out, it was one of those 12 yard triple tandem dumptrucks and the driver misjudged the turn and ran over the hood of her car with his back wheels (all six of them). It not only crushed the front of her car, but deformed the engine block too. She wasn't hurt, but it took six months for her to recover from it mentally. She called me on her cellphone while she was still in the car, and was unable to put a coherent sentence together. Fortunately, the cops called about 10 minutes later and explained the situation.