The OP's original post looks exactly like an issue with trailer brakes, if I had to guess, SURGE! I'd speculate he had to tap his brakes going through that intersection and his trailer did a little more than that, sending him into that post.
Surge brakes are tough. I am pretty much over them. I like to manually operate my trailer brakes rather than leave it up to chance inputs at the hitch. It takes a lot more focus, but when your trailer gets squirrely a little nudge on the actuator and it just straightens right out. Otherwise you need to get on trailer brakes in emergency stops, and sometimes going downhill, but you still want to rely on your engine.
OR, his trailer had no brakes and needed them, because it got squirrely side to side and pushed his truck off the road.
Either way, a lot of these problems come to trailer brakes either engaging when they shouldn't or not engaging when the should.