Are people mounting from the ground? Or maybe they are landing to heavy in the saddle when they mount? OTTBs are VERY sensitive horses. You have to lightly sit into the saddle. If you just jump up and land on their backs they will rear.
Also is she in season? Her ovaries might be sensitive.
The only way you will truly know is to have a trainer. But rearing 9 times out of 10 is a sign of pain. I suspect their is kidney or ovary pain. You say she is healthy. Has a vet done a complete work up on her? Because there are things a vet will see that you would never see.