At first, I thought the biker tookout a gun from his waistband.
In CA, I'd have to say it's both... 50/50 (with video evidence and/or witnesses)
Without vid... the biker would be at fault for following too closely.
That's what happens when cool heads DON'T prevail... would put yourself against a car in any situation? Guess he didn't learn the "rule of lugnuts"?