Maybe a crosswind, or maybe he was looking elsewhere and began to edge over and then swerved back to quickly.
I believe the lesson to learn here is that you should compress the suspension whenever you tie down a vehicle. This would have been about 1 oscillation if the suspension weren't allowing the load to pogo back and forth.
I'd say he probably bit a chunk out of that seat cushion!
What did he do wrong? Let's count the ways...
1. Trailer was probably well over max GVW
2. Not enough tongue weight
3. Pulling a load like that with a SRW
4. Failing to tap the trailer brakes, assuming he had some.
A cross wind probably started it. Combined with something like lack of sufficient tongue weight probably got the oscillation going.
Another is not securing the suspension and just tying the axles down. Note the large truck movement...the truck is a Mog with a significant amount of suspension travel, and because its a radio truck box it had a large profile to catch the wind.