My OTTB rears when being mounted

I have a 9 yr old OTTB who has been off the track since 2008. She is a pretty easy going mare but is a little high strung. I have only owned her for just under 2 months and she has been at the new barn about a month. I have been lungeing her daily and I ride her a few days a week. She reared the first time I got on her after I moved her, I pushed her forward and our session My sister rides as well and is there with me everyday of training. My horse has reared on her 3 times (does it every time my sister mounts). She did it to my husband as well. I will reiterate that SHE DID IT TO ME ONE TIME AND STANDS FINE WHEN I MOUNT HER!! I have checked her back and she is showing no signs of soreness. I have also checked saddle fit and seems fine. I think if it was saddle fit or soreness she would do it all the time not just everyone but me. Does anyone have any helpful hints to try to figure out the issue? I have trained horses but never had an issue like this before. I love that she is great for me but I also want others to be able to ride her.

Originally Posted by TheAQHAGirl     Possibly more ground work. You just got her a couple months ago, she has no idea who you are and what to do. Try gaining some trust and respect. Be patient and understanding. Did she just come straight from the track when you got her? (Whoop, just saw that she was off the track in 2008) Did her previous owners do anything to her after she was off the track?

Originally Posted by MickeysMama     The previous owners did some work with her except the one who I bought her from had not worked her in a few months due to falling off when she reared. She does not rear on me. I spoke with the woman who rode her before the woman I bought her from and she said that Mickey had not had this issue before.

Originally Posted by MissColors     The horse I've really known to rear at mounting had a kidney infection. You might want to get the vet out to do a urine sample. She might also be pregnant. I've heard of other horses doing that. Could also be your bit. One of the boarders at the barn. His horse rears because of the bit he uses(he just doesn't realize it). The possibilities can be endless. Best bet is to get checked out by a vet and then to get involved with a trainer. Posted via Mobile Device

Originally Posted by themacpack     So this horse has a history of rearing..........?