I suspect that it's not a difference in your riding either, but it would be worth having the possibility eliminated. The only thing that makes sense to me is that it's some sort of associaton from his previous training. If that's the case, time and repetition are the cures.
He's come a long way in a short time and looks spectacular; I wouldn't get too discouraged over what's really a minor traiining problem.
Its just frustrating, i've trained so many different horses, and I've never had a horse speed up while not getting heavy(that is easy enough to fix) so its just a new adventure, and that is what I love about training. I guess until it stops raining, and the ring dries out, ill just get to ponder it. ;)