The guy got punched because he was distracted by the cop. If you watch closely, just as the two meet up and are about to fight, the "apparent" instigator looks to his right at the approaching officer. Terrible timing on his part, as the other guy throws a punch at just the right time catching the other guy off guard.
One other thing that I caught (and is why I used the word "apparent" above) is that it appears the guy standing outside the car is the asshole. (Well, he is but pedobear with me.) The aggressor questions the driver about his road rage. It leads me to believe that the driver may've done something fucktarded to the aggressor who then let his inner asshole come out and approached the car.
Someone above said he should've never stopped. Absolutely right. If you stop, you can be held accountable, as well. Especially, since he obviously had his son in the car. Fuckin stupid.