I got Spike when he was 5 and I was and I was 21 (sigh). He was very green, but a pleasure to work with most of the time. Except when asked to canter. Then, he was a bonafied, certified crow hopping bucker.
The guy who had him before me was not a horse person, just the boyfriend of one. He didn't like trotting around an arena so he would take Spike out on the dirt roads and try to get him to gallop by bouncing up and down, clapping him on the sides and smacking him in the butt. Horse would go about 6 paces before putting his head down and dumping the guy.
So, every time I asked him to canter, he would eventually buck me off.
It was a fear issue with him. Every time he was told to run before, it felt like he was being chased. So we had to work on being able to go fast while remaining calm and relaxed.
I spent a Lot of time with him, and found him to be very willing to do what was asked of him.
The turning point came eventually. We were heading down a nice long grassy laneway when I asked him to canter. As usual, he soon put his head down and bucked me off. I landed on my face so as not to mark up my pretty hands. As I picked my self up and wiped the blood from my nose, I watched him buck away into the field. All of a sudden he stopped, turned, and walked right back to me. It was as if he was saying "I don't like this, I don't want to do it anymore."
I simply grasped the rein and mounted up again. "Come on, let's try again" I said. And we did it. Beautiful smooth canter all the way down the laneway, and all the way back. He has never bucked me off since.
Can't explain it, really.