This included a literature search through the Cornell Vet School library. You will never get anybody who can tell you exactly what is wrong. Therefore, the bottom of the coffin bone is not parallel to the ground. The horse is therefore standing to some degree on its toe. Appearance and Onset Age at clear manifestation is about 9 months.
Problem worsens for two to six months, then stabilizes.
Onset is probably earlier, but only becomes obvious to the casual observer at about 9 months. It doesn't appear to be caused or mitigated by anything in the horse's environment. The foot takes on a dished profile. It is upright and boxy. The heel grows faster than the toe, and the rear bulbs of the hoof are round and full.