Okay, now that I am back on my laptop, I can type out my response:
My first thought is pain. Either the pony's back is sore, the saddle doesn't fit, or something is up. I would get a reputable vet out to do a once-over - probably not a bad idea just to make sure her joints can stand to be jumping as well.
My second thought is: What kind of equipment are you using (bit, bridle, saddle), and does each component fit properly?
My third question is: How, exactly, do you mount up? If you are using the saddle a lot to mount up, and the saddle doesn't fit well, and/or she's back-sore, she could be rearing out of pain.