It makes no difference why or how she got this behavior, she needs a reason to change. More than likely, she spooked in the cross ties one day and broke free.
The solution is simple.Take a long rope, place the middle over her barrel, cross the ends under and through her front legs and feed them up through her halter. Tie the ends as a cross tie. When she pulls back, the rope will tighten around her middle and she will jump forward and find release. The horse learns on her own that rearing and pulling back does not make her comfortable.
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