I've always been told that changing leads is a sign of unbalance. I would recommend spending time at the trot doing work to balance him. When my OTTB had this problem, I spent lots of time trotting on a 20-meter circle, making sure he would hold his inside shoulder up and step under with his hind.
I'll leave the rest of it to someone with a bit more experience, but this is what I've learned from my horse.
I agree with Hedgie, he's doing this because he's unbalanced. Just about every "problem" originates from flatwork, work on a lot of hind end building - circling in/out, haunches in/out.
Really concetrate on keeping his hind end to the inside when you jump and start to feel him swap leads. Do a haunches in approaching the fence to keep him balanced and engaged. My friend's horse loves to swap back leads after the jump and in the corners, she does a "haunches in" and keeps him from swinging his hind end out from under him, which is essentially causing the swap. Works like a charm. :) Posted via Mobile Device