It honestly looks like anger. The semi is riding in the passing lane, not passing, going the speed of the right lane, with room to fit into the right lane as well.
Based on the fact the guys brake lights are on as the video starts it seems like he has been trying to get into the left lane only to have the semi block it over and over while refusing to get over and refusing to pass.
Is this guy an idiot for brake checking? Absolutely. Is there some fault of the semi driver, absolutely. It is illegal to ride the passing lane if you're not passing. Especially if video shows you were riding at same speed with room to get over the entire time as well.