Well I get on and we are tracking around the arena at a walk to get warmed up. The instructor asked us to get up to a working walk. I squeezed my legs around his barrel and he picked up the pace. Then she asked for the trot. Again, a squeeze from my legs and this time he pins his ears back, his body gets tense and he's flicking his tail like he's agitated. I thought maybe he's super sensitive to leg pressure, so I eased off on the pressure. He stops dead and starts backing up and sideways, tail still flicking. I wasn't pulling back on the reins at all, but he just kept going backwards. The instructor said to give him a more firm squeeze which I did. He responded with even more tail flicking and backing up.
I thought something was wrong with his tack. We checked his saddle, pad, bridle and nothing was wrong. They had used him earlier in the morning with no issues.
Since we only had 40 minutes left, I took him back and saddled up the horse that I usually ride. His ears were forward the whole time I lead him back. When I took his bridle off, he was nuzzling me. Strange....thought maybe he just didn't like me which is why he was being difficult.
Sorry for the novel :) My question is, what do you think was causing him to back up when I was trying to send him forward? Also, next time if he does this, or another horse I'm riding does this, how should I counteract the behavior?