My guess as to why the rider did it... he was following for a while waiting on this driver to MOVE OVER. For the biker to be that aggressive, he was probably the one initially tailgating and/or flashing lights trying to get the car to move over. When the car didnt, thats when he came around and messed with the car.
This is why you drive "right" the guy in the car was a jerk for not moving over. The fact he cared more about his car than the biker is an insight to his selfish mind set.
That being said the biker is a complete moron and basically got what he deserved.
You are right, the car should not have been just cruising in the left lane. We see this a lot in the USA. The left lane is for passing only. Yes the biker had all of the gear on. Can you imagine what would have happened with no gear on. But on the other hand the biker should have been driving defensively.