My thought is its something pain related. My horse is usually a perfect gentleman, but when he's in pain- watch out. When he's hurting, he becomes truly dangerous (biting, rearing, running you over, kicking, etc.). That's just his way of saying he hurts. If you correct him, he escalates the behaviour and becomes aggressive. I would get her teeth checked first, and watch her closely to see if she will show you where it is.
A few weeks ago, my boy became aggressive almost overnight (to the point of biting me twice). I had the vet out a few times, did bloodwork and x-rays, and found out he had an abcessed tooth and was in extreme pain. If I would have listened to the vet saying he was fine, it could have killed him. Really watch your horse. Does she seem angry, resilient, fearful, or does she look like she's in pain? If you really watch them, they will tell you what their feeling. This will give you a good idea if its pain, behavioral, or training method.
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