my mare does this on any surface when she is being lazy, actually most of our TWH's do it.:) When she dog walks she is more swingy in the back end and tends to hit her back hoof on her front one before she lifts it and moves if forwards. She also does this if she needs a trim, but if she needs and trim and is doing this she is also trippy at a faster speed too. It probably means that your horse has a decent back end because the ones who dont dont tend to swing their back legs far enough forward to run into the problem.
Try waking him up first and asking for speed, if he wakes up and you go back down to a flat walk and while he is alter he still does it, he likely needs a trim.