Well, based just on information already provided in this thread: we don't have all of the video. We don't know what took place before the recording started.

We do know that he has his sunglasses on and at least some CBP agents will insist that you remove them and keep your eyes visible and exposed even if that means the sun is shining directly in your eyes.

He also kept picking up his camera and at one point, he picked up a clear plastic bag with some kind of brackets. His actions demonstrated that he had been told to keep his hands visible and with nothing in them at all times: he waggled his fingers in response to something an agent said to him on a couple occasions after he put the camera down and was clearly told to put the brackets he was assembling down.

Clearly that was enough to justify smashing in his window without warning. If that really happened. We don't have the testimony of a qualified CBP forensic video analyst.