So when the guy approached the man who ended up on the ground didn't feel threatened?
For all he knew the guy stepping back was doing so because he was too close to kick him in the face before he could get up and would kick him after a step back. I've seen that happen several times over the years, maybe the shooter had, too.
Once it was obvious the man who did the pushing was intent on using force (in fact had used force right off the bat), you can't really say the man who was already physically attacked should be able to make the split second decision more force wasn't going to follow.
The shooter would have been a lot better off not arguing over the parking spot given the fact that police seldom if ever enforce restrictions anyway or at least done his commenting walking. Once he chose not to, though, what followed doesn't look nearly as clear cut as you make it out to be with 20/20 hindsight.
If you're the one who may be a second away from being kicked in the face you wait and see but don't expect anyone else who is shoved to the ground to see it the same way.
If I were knocked to the ground by a feral, I'd be worried about any nearby "kickers" ready to join in.