"We need to get the motorcyclist's side of the story to see what led up to this incident. We're not sure what happened farther back."
- California Highway Patrol
As the CHP quote above indicates, something must have led up to what you see on video, and that something was evidently enough to provoke the motorcyclist into lashing out right there on the 14 Freeway in Santa Clarita.
Pure speculation: Perhaps the car driver "didn't see" the motorcyclist and cut him off (not to excuse the motorcyclist's actions).