These people I know asked me to work with their paint mare, so I obliged. She's got lots of problems, mostly she's just nervous about the world, which I can handle. We've been working on that for sometime now, and she's doing great. She wasn't broken, or even really handled until she was four (she's now 6), which I think is where quite a few of her problems come from. I've been riding her, mostly getting her back into shape, but also working on flexibility, moving off my leg, things like that. She's been coming along very nicely.
Anyway, back to my problem. Today I decided to lunge her so we could start working on her headset, since she runs around with her nose in the air like a pig. Her owner said she had been in side reins when I started with her, so I naturally didn't think there would be any problem. When I started with her, we basically started from the beginning, doing ground work to establishing me as the boss and what not, and I had problems with her rearing during lunging and our other groundwork that we resolved. Anyway today I warmed her up on the lunge line- no side reins. No problem. I brought her in and put on the side reins on the last hole, nice and loose, so she could get used to them again, she was fine. I backed up to start her walking and she reared straight up and flipped. I had no idea where that came from. She got up right away, and I let her stand there and calm down, she decided on her own it was ok to take a few steps and then remembered about the side reins and flipped again. She did it one more time, I finally walked up to her and led her a few steps to show it was ok, (which was scary, and maybe not the best idea) then removed them, put her away, made sure she was all right, fed, and left.
I was too frightened and shaken to do much else.
What did I do wrong, and how do I fix it?