"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).
Whatever the case, the Los Angeles Times reports that the motorcyclist kicked the side of another car while on the freeway. In turn, the driver of that car swerved into the motorcyclist (but evidently didn't hit him hard enough to topple, as the motorcyclist got away) and then lost control, careening the other way, bouncing off the concrete barrier and slamming into a third vehicle.
There were no fatalities, and injuries apparently weren't very serious, though CHP said, "[T]his could have been a horrible tragedy. More people could've gotten hurt."
Whoever it is, the motorcyclist is one lucky rider. It takes a certain proficiency to kick another vehicle while riding and remain in control after being sideswiped. According to the National Highway Traffic Safety Administration, 88,000 motorcyclists were injured - and 4,976 motorcyclists killed - in 2015. This rider could easily have become such a statistic in 2017.