Koda didn't like that.
He really didn't like that!
He went to town, crow-hopping for all he was worth. I grabbed his reins with a death-grip. I wasn't about to let go of him until Kid 3 was safely off! She executed an excellent emergency dismount and landed on her feet. She thought the whole thing was kind of fun and was wishing she had been settled into the saddle so she could have ridden it out. Kids! (sigh) We got Koda settled down and then Kid 3 hopped back on and put him to work with no further outbursts.
Then it was time to fix the hole in his training. I started by getting out my biggest, bulkiest set of saddle bags. I tied them securely to the back of the saddle, but with just a bit of play. I wanted the bags to slide around slightly while he was moving to help desensitize him to feeling weight and movement behind the saddle. The way he took off running around the round pen was a pretty good clue that he wasn't entirely comfortable with the situation.
The next day, I took it a step further, and tied an empty gallon bleach container to one side of the saddle. He didn't like that either. In fact, he tried to cow-kick it as he was running around the pen. Then I tied an empty gallon OJ container to his other side, just to keep things balanced.
Once he was relaxed enough to follow my cues to start, stop and turn, I switched gears and left him in the round pen to come to terms with his situation. It was good that I had put my old, beat-up saddle on him, because he rolled twice in an attempt to remove his unwanted burdens. After a couple hours he seemed to accept his lot.
The next step was to recreate the original incident. I asked Stable Boy to come out and hold Koda while I first perched on the back of the saddle and then slid back to sit behind the saddle. We did this at a stand still and then Stable Boy led Koda around while I repeated the steps. He was a still a little tense at first, but then he relaxed and did great! No crow-hopping at all.
Since then, we've gone back to trying to help him relax and improve his responsiveness to cues at a higher speeds. We'll have to see how much improvement he can make in the next few days--he's scheduled to go home this weekend.