I am out in the arena now. There is a beginners lesson going on, and I am walking up along the rail. I ask for a trot, and he tosses his head like usual, but will not trot. So I let him walk a little farther and ask him to trot again, and this is where the title comes in. He EXPLODES at me. He gives a very nice buck before taking off into a canter down the rail, tossing his head sky high as it happens. I am very proud of myself because although my right foot came out of the stirrup, I keep my reins in one hand, my crop in the other, I do not fall off, and I manage to get my other foot into the stirrup all while actually slowing him down. I remembered to do half halts because he is an OTTB and will only lean in to constant pressure, and when he is still jittery I turn him in circles till he calms himself down.
What was the cause of this explosion? His saddle was about three inches too far back. In all of his moving on the single tie, it slipped back while I was tightening up his girth, and he obviously wanted to tell me about it. So I dismount, go to the cross ties and re-tack him up the correct way. He is perfect again, and although I am too nervous to ask him for a trot, I let his reins out and he trotted a little on his own. My main goal in getting back on was to let him know that for future purposes, bucking me off was not an option.
The moral of the story? Check your saddle position. What I took from it? Practice tacking him up single tied on a day that I don't plan on riding.
On the flip side, I was told by many of the other boarders that he had been sour while I was gone. He really does appreciate my company.