Ok an update! Heard back from the trainer out at the ranch. He did injure his back and apparently it's super sore.
So I'm glad I took the time to come here, bounce if off you great folks, talk to friends, etc. Going to work him on the ground this weekend, give him a bath, take it easy.
I heard his back is so sensitive he jumped. It makes sense then that A: he needed me off after that first spook and round of bucking, B: I got off and I bet it STILL hurt and he just couldn't quite figure out what was going on.
I think I"m going to look into another saddle. The owner bought these saddles that seriously weigh like 40 pounds, sturdy as hell but I'm thinking I'd like my own saddle that isn't so heavy (hard to get up on this guy, he's a big boy). I'm sure a couple of stiff jolts with a 40 pound saddle and my 170 pounds was enough to injure him. Poor guy :(
Still don't know what spooked him but I think we solved the mystery of the continued reaction.