What would cause a mare to buck and rear?

Answer 1

My best estimate without seeing the horse or knowing anything about it that the mare is either in heat causing her to be cranky or agitated, the mare's tack is rubbing her and she dislikes it or she has pain with the tack or the weight of a rider. Now if the horse isn't ridden it could be out of boredom or if the horse is in the field doing this than its being playful, now if your leading or grooming her than your agitating her with something YOUR doing or something or YOUR tack is bothering her. Now I only capitalized YOUR because its never the horses fault this is the horses natural instinct to do this.

Answer 2:The other reasons could be something that you or the previous owner failed to do with her earlier while breaking her to ride, such as not training her on both sides instead of one, or trained her more on one side than the other. Possible reasons, expanding from above, could be:

• Misfitting saddle
• Wrong type of bit
• You're pulling back too hard or too much on the bit
• You're sitting kinda awkward in the saddle
• Skin infection or tenderness in her withers and back causing her to not like the saddle on her back
• She's just in a cranky mood
• She's not as broke to the saddle as you thought
• Cinch too tight
• Bit cutting into her mouth
• Teeth need to be floated and the bit is simply aggravating the pain
• She's bucked someone off before and knows she can do it again
• Fly bites on her back is causing her pain when you put the blanket and saddle on

The list goes on. You may have to consult a few people, like an equine vet, a horse trainer, horse behaviorist, etc. to pin-point what's going on and why.