It is hard to tell who started it in this road rage incident, but we clearly know who ended it. Watch the blue car on the right at the beginning take issue with the black truck for whatever reason. After he tracks him back down the two drivers exchange words until the driver in the blue car throws something (A bottle?) that hits the truck.
From there it looks to turn into the classic road rage story of escalation after escalation until someone gets hurt. In a perfect twist of fate, a cop was exactly where he needed to be to witness the road rage before it got out of hand and turned around to put a stop to it. Wouldn't it be nice if there was always a cop around during these incidents?
Related: The Instant Traffic Karma of 2017