What causes bucking under saddle and how do I identify the root cause?

Bucking under saddle is one of the behaviors that most demands correct diagnosis before correction, because the same behavior has multiple causes that require entirely different responses. Physical pain is the most important cause to rule out first and the most commonly missed. A horse bucking from back pain caused by a poorly fitting saddle will worsen under any training pressure and may escalate to dangerous behavior if the pain is not addressed. Signs that pain is involved include bucking that is consistent and predictable, that intensifies when the girth is tightened, that occurs immediately when the rider sits, or that is accompanied by other pain indicators such as ear pinning, reluctance to be saddled, a tense back under the rider's seat, or a shortened stride. Have the saddle fit evaluated by a qualified fitter, have the horse examined by a veterinarian if the behavior is new or escalating, and assess for gastric ulcers if the horse shows additional sensitivity around the girth and belly. If pain is ruled out, evaluate the training context: does the horse buck only at the canter departure, suggesting a weak or unbalanced transition? Only in one direction, suggesting a stiffness or suppleness asymmetry? Only when asked for collection or more engagement, suggesting the horse finds the demand too difficult? Each of these patterns points to a training gap rather than a pain issue and calls for building the horse progressively through the specific challenge rather than escalating pressure through it.