Yup, there appears to be something missing that we're not being told here. People just don't go off tap for no reason at all, unless they're under the influence of Drugs, or a predisposed position (mental illness). Though the attacker didn't seem to fall into either category.
I wouldn't be surprised in the slightest the bloke filming it, is one of those A-holes that we all come across time 'n time again. That thinks they own the roads and will do whatever seems to please them. If that entails trying to piss other drivers off, so be it..