You must not have seen the video. After frisking him, the cop can clearly be seen inspecting the back of his head. Then as he's walking past the motorcycles, when paused, you can see a mark several inches long on the back of his head.

Remember that paramedics treated him at the scene.

I'm not saying what he did was right, I'm just saying that the video clearly showed that he had an injury to the back of his head.