Never had them myself, so can't be sure, but I would imagine it has to do with dead skin piling up. A bit of a healthy workout for your soles over some rough pavement should keep your feet fresh and in check.
Mine cracked bad from high altitude last summer , and I when got 'em healed de'feets were better than ever . It didn't take long . I used a balm of beezwax and olive oil , and a couple steps in the mud/clay ... No soap , no washing . Just let the mud dry and then dust off . And then massage lightly with oil .
generally, any crack in the skin is caused by overly dry skin (higher altitudes almost always have dryer air). moisturize a lot, put something on the crack to make sure it doesn't get infected, don't use harsh soap, as it tends to dry out skin. and vaseline or bees wax will help to keep moisture in the skin until it's healed
