As for why hit her... I wasn't there for that particular incident, but my partner HATES driving with her mother. She flips people off, chases them down at 100 mph (yes 100 mph in a minivan), cuts them off and brake checks them, forces them almost off the road or into a guard rail, etc... does things far more dangerous than a punch.
If someone almost ran me off the road (LITERALLY) I'd want to deck them too, even if they were twice my age. But since I'm an athletic 31 year old 6'2 inch woman, if I socked someone 55+ years old (even after they almost ran me off the road and after they asked for it) and I got shot they'd probably consider me the sole instigator.
I'm not saying that's what happened in this case but I can see something like that happening.