I had a horse that would do the same thing...I had turned the third barrel about halfway to the timer and she just blew up (I managed to stay on), The anouncer gave me a score of 74...
Im not sure why she did it but she would do it once and awhile at home too, After a few times I could start to feel her getting ready to buck so I would try to make her run out of it... if she still got a buck out on me I would make her pay for it by making her do extra work- circles, running, etc... she learned that it is more work to buck then to just do what was asked of her. It took some time though. Good Luck!!!
It's not the will to win, but the will to prepare to win that makes the difference.
- Paul "Bear" Bryant (Former college football coach)