So now you know about her, here is what happened. I had decided that before any more rides I would basically start over with her on ground work and desensitizing. She isn't spooky at all, but I have seen how that can make a horse have a better attitude and in general be a safer ride. It was a nice sunny day for once and she had been doing great at moving from pressure and listening to me on the ground. I decided to have just a small ride. Now the ingredients for this situation are as follows: She was already really rusty when I got her but I could tell at one time she was fully broke, It had been about 2 weeks since I had handled her and ridden her, and I had put a back cinch on her for the first time since I have had her. Now I did some research on uses of the back cinch and found that when it is pretty snug it actually holds the saddle on the back of the horse better. Since my new saddle has full rigging I knew I needed that. So I went ahead and cinched it up pretty good. She didn't bat an eye and showed no indication that it bothered her at all. I did some lunging, bending, driving etc and she did great. Then I decided to ride. Before getting on I tightened both front and back cinch pretty good. She stood there as happy as punch. As soon as I was in the saddle and even before asking her to move out she started in with a good old fashioned bucking. Not crow hopping or pig rooting, just the classic rodeo-popping up and down like a rocking horse from hell-bucking. I didn't know why it was happening and thought maybe the cinch, I bailed. I know, I should have stuck with it but it surprised me so much since she was giving no indication that she was going to do it at all. When I landed and watched, I noticed she did keep bucking for another 10 - 15 sec before stopping. When it was over, she didn't act all upset or scared, just walked around. And when I came to get her she stood quietly like "So, nice day we're having." I didn't take the back cinch off, or loosen anything. I did lunge her in all gaits for about 5 min or less. She was totally fine and not stressed at all. I then got back on, but this time more slowly, with her head flexed over. As soon as I was on, I did not give her head but immediately had her move her hind. I did this for about 2 min and then let her head go to see if she would walk forward, she did and seemed ok, but I did have her give her hind several times more as soon as I felt any resistance. I was able to do a trotting circle with her a couple times and decided to call it good.
So my question to all of you is, What the heck? Was it my fault with the cinch? Is she just going to do that randomly? Like I said she showed no signs of being upset at all before the bucking. I do plan on taking her back to square one since we don't have any kind of relationship yet and to get her really tuned in to me before riding again. I am happy that I was able to get back on. Any and all advice is welcome.