I agree with everything above as well. I occastionally have this problem with Cinny when he gets bored with an exercise or doesn't want to do something, except that he will rear 80% of the time and give a buck the other 20. I found that it is best to never stop asking until you get what you want, even if you end up on your rear on the ground. It will only teach her that she was able to get her way by doing that.
But I do have another question. Does she only have hissy's while riding, or does she have them on the ground, or even in her stall? The reason I ask this is because (although this isn't super common but it happens) your mare could have a hormonal imbalance as well. My friend's mare used to have hissy's out of nowhere once or twice a week, except when she was "in season." She busted halters and leads when tied, she busted bridles, she is a higher level dressage horse and my friend was losing money at shows because if her mare was having a "bad day" they'd have to scratch all her classes for fear she might explode in the warm up pen or during her run. A couple of months ago they did some bloodwork on her to check out her levels and her hormones were way out of wack. She's on therapy for it now and has completely changed back to her old sweet self.