It’s because they suddenly switched from going downhill to going uphill, and didn’t correct their driving to account for that. Going downhill, they were likely accelerating to get past the traffic. So the boat was actually being pulled downhill. Then when the slope began to change, they stopped accelerating because they were past the traffic.

This means the boat’s inertia was suddenly pushing the car, rather than being pulled by it. If they had continued accelerating up the hill, (and thus, kept pulling the boat) they would have been fine. But since they were starting to slow down, (and the boat was pushing them from behind) they essentially got PIT maneuvered as soon as the back end wasn’t perfectly straight.

Either way, the correct solution would have been to accelerate. But inexperienced trailer drivers often panic and brake as soon as a wobble starts, which only makes it worse. Imagine the trailer being held by a rope instead of a ball hitch. You’d want to keep tension on that rope constantly, so it doesn’t go slack. Because if you put slack in that rope, the trailer will try to veer off in one direction or the other, and you’ll be along for the ride. That’s essentially what happened here. They went from pulling the trailer to being pushed by the trailer.