Facepalm

They were going downhill towards that bridge. Once they reached the lowest point (the bridge), they would naturally start slowing down and the boat would be pushing into the car, making the connection point want to go in a perpendicular direction from where the vehicles are going + at the same time the wobble gets worse because of the deceleration.

If they had been descending slower in a controlled manner, then they would have had enough power margin at the bottom to accelerate out of the pickle, but instead it seems like they sped downhill at max power, so they had no margin left. The vehicle was probably more than adequate for the job, but speeding down hill is usually not part of the job description of towing.