Major cause is poor shoeing. Farrier
shoes too tight and doesn't allow enough width of shoe at the heels to
support the hoof when it expands. Over time, the heel just keep getting
narrower and narrower. The answer is to allow enough shoe on the sides of
the heels to allow the necessary expansion. Very difficult when the horse
is a shoe-puller, because it is that very extra bit of shoe that gives the hind
feet something to grab onto.
