Rule out pain

If an action causes pain, your horse will do what he can to avoid or minimize the pain.

some common behaviors that can be pain-related include not standing still for mounting; bucking or rearing; raising his head during bridling; pinning his ears during saddling; or not standing well for the farrier.

If your horse has any of these behaviors, explore if he has pain in his back, mouth, feet, or joints as a possible contributor.