I didn't really understand the either. At first I assumed the guy with his shirt off (the black BMW owner?) was having a go at the white BMW owner for no reason. He was being the most aggressive, especially reaching into the other car, and the other guy was just trying stop him opening the door/getting in. Then the camera dude walked around and things started getting heated...The white owner was kicking his own door aggressively, probably to hit the black owner, and I noticed his right wing mirror was broken. It could be that the white owner started all of this, as he then showed his true colours, with a child in the car, by recklessly pulling away and ramming into an innocent Mercedes.
That's how I saw it anyway!