The boat trailer started to weave and jack-knife on deceleration. I can't really say what the ultimate cause was, but this sounds a lot like the trailer brakes were not working correctly. A larger boat and trailer need working brakes or it is an accident waiting to happen. Please people, make sure your trailer has brakes and that they work!
Straps may not help all situations but you never know when they will plus you have an obligation to others on the road. After following my boat being trailered and watching it move over a road with some not so visible wallows I always tie the transom down. I also had a boat come off the ball over railroad tracks, you just never know and it only takes a second.
This same thing happened to my parents about two months ago. They got in a wreck and left their boat in the middle of the freeway while passing thru Bakersfield. Thankfully no one else was injured. My mom broke her pelvis and pops fractured some ribs, but they are healing up.