I'm going to be bold. I believe that it's both. The horse is being taught ten different ways how to do something. One rider takes off on him. Another wants him to wait, and then you're left working with him just trying to push you around.
I agree that he isn't the best lease for you..
Then your technique is interesting.. taught reins and slack when he's good. Well what about starting with slack and then relaxing until he tries something.. then correcting him and going back to slack.
Because constant pressure = horse will try to get away from it. That may be why he was rearing, in accordance to something else physically related.
"Strength is the ability to use a muscle without tension"