Worst case of road rage I've seen.
Looks to me like a bad case of road aggression mentality.
My guess is that the maniac thought the 'good guy' brake-checked him (I believe I heard something about him briefly applying his brakes when the maniac was behind him). The guy must have a real hair-trigger to go off like that.
Actually a lot of Jeckel and Hyde types out there driving. Personalty completely changes when they get behind the wheel.