It's a risky and illegal move for the motorcyclist.
This isn't the first time we've seen a motorcyclist purposely strike the mirror of a tesla and it probably won't be the last. In this instance, it all happens so fast (and the video quality is quite poor) that we can't make out the license plate on the offending motorcycle.
Typically, when we see a video like this one, the first question is what did the Tesla driver do to deserve this? Well, in this case, the driver of the Model 3 thinks it was partially the fault of Autopilot. Here's the question/answer detailing this in comments on the YouTube video:
Question: The only thing I can think of is that you were too close to the lane on the right. If you were using autopilot, it usually does that. When I split lanes, mostly Tesla cars would be close to the dividing lane and it makes it tough to pass you. Or, as someone else already said, you had prior encounter with that fella. Did you catch his license plate?
Answer: I tested that Theory of Autopilot, and I think you are right, and No the Cam was not able to get the license plate, it was not clear, it happened so fast before I could realize what had happened he was far off, but thanks now I know what happened.
Video description via Goddy Osas on YouTube:
A motorcyclist broke My Tesla Model 3 side mirror. I would like to ask why? (if I had the chance) What did I do? Is it an offense to own a Tesla?
I actually stiched together both the back camera and right side camera together, if you noticed. More over, the clips I had prior to this video the motorcyclist was not even in view.