My OTTB rears when being mounted

Seems like the OP has all the answers when it comes to checking stuff, and she is very experienced, so I'm really not sure what anyone here can add

I ruled out pain and saddle fit with the help of a vet and trainer. I began to think it may be fear of the mounting block but I think she would do it everytime not just other people.

Is the rearing only at mounting or has/does she do so while under saddle after mounting?

I just need to know if other than the obvious if anyone has anything else I can check into. The only other person who rides her is my sister and she is an experienced rider. I have been racking my brain trying to think of something outside the box so to speak that could be the issue.

Well then why don't you tell us why she is rearing?

I am still saying its pain. It could be anything from a sore mouth to kidney infections. Pain can be in more places then just the back.

However I agree with others. You are so positive its not pain related(which I would bet you anything it is) then you obviously know what is causing it and you are in denial.

If by chance its not pain related. I suspect its you. You are standing beside her when she does it? You mentioned she broke a bridle today which makes me think you are holding her. If that's the case are you sure its not your body language? Or maybe you are holding her in a way that scares her?

Only while being mounted.

So she was checked two months ago or two weeks ago? Were both "checks" done by a professional (only asking as you asserted in your OP that you had checked her for pain and saddle fit but have also said that she was checked by a vet)?

When exactly does she rear? As they mount, as in while they are trying to get on? As soon as they sit down? When they ask her to move?

What happens after she rears? Do the others get off or are thrown off? Do they get back on? Is she put back to pasture? Or is she disciplined right the on the spot?

It sounds like it has become a habit she has learned to get out of work. She is testing the other riders. She tested you and you corrected her, so she doesn't do it for you. Only having her for a few months, it is typical that the horse tests you and others out. She is seeing what she can get away with and if she can get out of work with it, like she probably did with the previous owner.

It is difficult when the horse acts up for others or just on certain circumstances. Either you have to recreate the situation or just try to correct it when it does happen with others.

I just thought of something. How are the reins held when getting on? Is she antsy to get going? Are you or the others holding the reins tight so she doesn't walk off? Do you turn her head towards you when you get on? Do you have her stand for a little bit after she is mounted? What I do with any horse I mount is turn their head towards me as I mount. Once mounted, I have them stand and flex them to each side several times. Having their head turned does several things. It's harder for them to rear. If they do act up, their butt would likely go away from me. It gets them to pay attention to what you are doing. Standing afterward and flexing gets them to not want to always take off and helps get them focused and listening to you. Kind of like a preride check list for safety. It can show if they are willing to listen or are full of themselves.

I have to agree about getting a trainer. Hands-on help is the best. Without seeing what is actually going on, we can only give hypothetical advice.
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She was checked two months ago right before I bought her and I had her checked 2 weeks ago to rule out pain...both checks were done by a vet.

She does it when the are mounting..usually once they are sitting. The reins are loose and there is no contact on her mouth. The previous owner fell off and did not get back on. My sister has stayed on each time and pushed her forward. She stands still and does not seem antsy at all when she is being prepared to be ridden. She does seem wary of the mounting block but I think she would rear on me if that were the issue. Like I said I have checked all the obvious possibilities so I am looking for the not so obvious.