My cousin's horse hated it when she groomed him. She wasn't rough or anything, but he would pin his ears back and snap at her. I didn't have any problems with him. Eventually we figured (we think) what was wrong: The only time she ever touched that horse was when she was going to ride him. I fed him and brushed him every day. (He was staying with us.) She would show up whenever, give him a good brushing, saddle up and take off. If he was in the pasture when she drove up, there was no way she could catch him. No one could catch him when she was around. I didn't have any trouble catching him - didn't even try to catch him - he would just walk right up, even if I had a halter in my hand. The thing is - he associated her (my cousin) with having to be ridden. Everything she did led up to being ridden. On the other hand, although I also rode him, I didn't just mess with him before riding him. He didn't know if I was catching him to ride him, or just to feed him, or groom him - or just to pet him.
Is the only time (or even the majority of the time) you brush your horse just before you ride him?