Just saw that on another website. Either he didn't check his mirror or has no idea how long his unit is. Or, as stated above, was just impatient. He did come screaming up behind the other truck.
I also feel bad for the trucker. What amazed me most was the truck moved very little and the driver kept it under control. See this every day on so many different roads. Just yesterday we were stopped at a light that had a left hand turn lane. Light turned green, my husband started to go but checked the side mirror as he always does. Some jerk came flying up the turn lane, cut us off, almost hit the front of our vehicle and then went screaming down the road.