Not an intentional reward :)
The horse doesn't care whether you wanted to come off or not. Fact is you did come off, and she got to have her little fun bucking fest without consequence. By the time you are able to get back on her, it is too late for an effective consequence.
I did come off my stallion when he reared one time. I was bareback so I just sort of slipped right off. Luckily I landed on my feet and somehow still had a hold of him. When we landed, I went after him like I was a horse eating monster and drove him backwards across the length of our arena. Needless to say, that was when he was three years old, and he did not try that again. Point is - I did come off, and had I not been lucky enough to be able to respond immediately, I would not have been able to give him an effective consequence for his action. Just this year, he has started experimenting with rearing again (age 5), but we have begun some new, pretty intense training for reining, and he has been learning to get on his hind end more - freeing up/picking up those front feet has been encouraged, and so he likes to test just how much would I like him to pick those feet up :) Again, he receives a consequence every time he does this.
I know my example is a different bad behavior and therefore the actual consequence is a bit different (I pull his head down and drive circles rather than pulling the head up), but I hope it was helpful in explaining the importance I was trying to express in immediate rewards/consequences.
Maybe you didn't reward her by falling off so much as you were unable to give a consequence to her behavior when needed. Rewards and consequences are both very important and go hand in hand. A reward can be the simple action of letting the horse relax and trot a straight line after successfully picking up the canter without bucking. Rewards do not always have to be a positive action - they can be the removal of a stimulus. I am sure you know this, but it is important for you to somehow communicate to them both when they have done right and when they have done wrong.
I hope you are able to solve your problem :) Just don't foget, with this age, when you solve one problem, they test you more, and new problems are presented. It is hard work, but rewarding. Good luck :) :)