I think it is mostly a myth. There are a couple of half-truths behind it, though. As has been discussed already, a lot of gaited horses are shod/trimmed with very long toes, which can certainly cause tripping. Also, “loose” stifles (i.e. a tendency for the patella to lock) is a very common condition in TWHs and their derivatives, especially the ones bred for the show ring. This can cause tripping/stumbling in the hind end. Proper management can minimize or eliminate that problem as well.
