At the start of the video, the bus is in the second from rightmost lane and looks like it's starting to merge into the rightmost lane, which will put it directly in front of the black truck. There's a white crossover SUV that is directly behind the bus and will no longer be directly behind the bus once the bus finishes merging.
The camera turns away unfortunately, then pans back to show that the bus has merged and the white SUV crossover which was behind the bus before the lane change is now trying to merge directly into the black truck, in order to stay behind the bus. The black truck then pushes the white SUV crossover back into its own lane.
Obviously what the white SUV crossover did was illegal. I think the most charitable interpretation of its behavior is that the driver didn't see the black truck.
The black truck driver could have rapidly decelerated to allow the white SUV crossover to merge in front of it, but I don't think it's fair to characterize pushing back on someone trying to force you off the road as "attacking." A more accurate description might be "defending." I probably would have done the same, since I don't like bullies and don't like to endanger the innocent people behind me by rapidly decelerating due to distracted/angry driver next to me.
This isn't the best video, but if you pause it and determine which lane each vehicle is in, it seems fairly straightforward. Do you have a different interpretation of this video, or perhaps a better video?
EDIT: clarified wording about "attacking" vs "defending"