It's hard to tell from the video, but assuming there is truth to the claim that the other driver was identified and their insurance company is refusing to pay, it's likely from the fact that the rider appears to be accelerating/decelerating to irritate the person in the BMW. Assuming the rider was accelerating while being passed he would have been violating CA Division 11 - Rules of the Road, Chapter 3. Driving, Overtaking, and Passing, Article 3. Overtaking and Passing.
Code:
V C Section 21753 Yielding for Passing
Yielding for Passing
21753. Except when passing on the right is permitted, the driver of an overtaken vehicle
shall safely move to the right-hand side of the highway in favor of the overtaking vehicle
after an audible signal or a momentary flash of headlights by the overtaking vehicle, and
shall not increase the speed of his or her vehicle until completely passed by the
overtaking vehicle. This section does not require the driver of an overtaken vehicle to
drive on the shoulder of the highway in order to allow the overtaking vehicle to pass.
Amended Sec. 9, Ch. 440, Stats. 1996. Effective January 1, 1997.
Amended Sec. 40, Ch. 724, Stats. 1999. Effective January 1, 2000.
So it could just be reckless driving all around.