Why has my horse started bucking when asked to canter?
The question is a little too broad to attempt to answer based on the information given. More information along these lines would be required: How old is the horse? How far along in training is it? Is this a seasoned riding horse, or is it a young horse just started? Have any circumstances changed to possibly cause this (new saddle, new rider, etc.)?
The answer at this point could include several possibilities:
1. If the horse is already trained and a seasoned riding horse, maybe there is a physical problem. Maybe the saddle doesn't fit properly and is causing discomfort, so the horse is reacting to this. Or the horse could be sore on some part of its body causing discomfort when asked to canter. Also check for lameness. Pain can cause severe reactions such as bucking.
2. If you are a new rider for the horse, maybe your cues are confusing the horse. Make sure you are using cues the horse understands. (Normal cues for a canter would be to collect the horse slightly with your legs and reins, rein slightly toward the outside, and apply pressure with the outside leg behind the girth. As you press with the leg, release the face so that the horse may go forward. A verbal cue such as a kiss or cluck may also be utilized.) You may need to get a horseperson friend, a local trainer, or a riding instructor to watch you ride and see if the cues are the problem.
3. Generally, undesired results come either from confusion or a lack of consistency and training. There are some rank horses, but most are just confused. Without knowing the entire situation, it is hard to diagnose.