If you watch the video frame by frame, shortly after making contact with the car, as the rider begins falling, you'll notice something in the hand of the rider, either reflecting light, or producing light. In the next frame, you'll clearly see a smartphone facing directly at the camera on it's way to the ground. In the next frame, it's almost at the ground, but still visible. Then, it's gone. I think it is HIGHLY likely to be the cause of the accident. A motorcyclist (or anyone driving anything on the highway) shouldn't have a hand preoccupied with something else.
Whether the phone was just in the hand, not being used and caused the rider to be unable to safely steer away, or the rider began receiving a call and glanced to see who it was, or the rider was actively texting before the incident, it shouldn't have happened. Just put your phones away, people. Focus on getting to your destination, otherwise you'll end up like this guy and your destination will be the hospital.