Okay so here's what happened. The abrupt braking caused the trailer to essentially jack-knife a bit (except vertically, not horizontally); the bolt the cotter pin feeds through broke, the hitch popped off, and the force of a large, heavy, steel 4-horse trailer carrying 2 horses and going around 65mph was enough to severe the chains and brake cables.
I've been hitching trailers since single digits age and can do so in the dark and in my sleep. In fact, some early morning launches were just that ha. Everything was in place that morning, and double-checked by another (standard procedure). Although I can't recall who it was that hitched that particular day. Regardless, it was all in place.
Yes it was scary. I still remember vividly one of horses looking at the truck as he passed us, wide-eyed and forelock blowing in the wind as the trailer careened past nose-up.
Fortunately, it occurred during an empty stretch of the heavy on-coming traffic and the trailer was stopped on the grassy shoulder by a small sapling - thank God for that sapling! - just before encountering the cement overpass wall several yards ahead.
Horses were fine; trailer was not as the driver of the truck attempted to first veer in front when it broke free to stop it, leaving a crater indentation in the nose. There was a quarter-sized dent in the tow vehicle however.