Well she could have a number of problems. Under the pain category... It could be:
-ill fitting saddle (expecting pain)
-her back could be out of alignment
-you could be tightening it too much to fast
Or it could be:
-she could have used to have to previous problems with an old owner, and it turned into a habit.
1.How long have you had her?
2. Has she always done this?
3.How does she do once saddled and being ridden? 4.What is her diet like, as in pasture, grain?
Just give more general information on her. Posted via Mobile Device
You need to find out the cause - alot of horses will react with "violence" when they anticipate pain or have a fear of it. Does your mare have back pain? Does her tack fit her properly? Do her ears go back when she sees the saddle? Could the cinch be putting pressure on sore spots along her belly?
Well if you can rule out pain, then this is classic spoiled horse behavior. All horses that have ever worn a saddle know that the saddle means work, if they can keep the saddle off they don't have to work. Solution? Make not taking the cinch more work. Have the horse in a round pen or on a lunge line, do what ever you need to to get the cinch on and tied (not tightened but tied so it will stay on). Slowly begin to tighten it, the first time the horse turns to nip or steps away send it for a couple of laps then bring it back and start again. Tighten it more, more bad behavior, more laps... I've done circles with Nate till I was sick (just the other night mind you) and after about 20 laps of the round pen he decided being saddled was less work.. If I don't have him on a long lead or in the pen, I just grab the reins or the lead rope and chase his hips in circles till I'm stumbling dizzy..
Then do your best to make being ridden WAY more fun than being driven from the ground...