Originally Posted by Pedigree Ann

Buckpasser had a problem with quarter cracks - he missed the entire Triple Crown while his hoof grew out after having one - but he still managed to win 25 of 31 races over 3 seasons of racing, from 2 to 4.

I copied this from a vet report:

"Quarter cracks are a common cause of foot lameness and/or decreased athletic performance in race and sport horses. They typically originate at the coronary band and continue distally. They are full thickness, extending into the dermis of the hoof, which leads to instability, inflammation and/or infection. Quarter cracks are painful due to infection and/or the result of instability caused by movement of the hoof wall posterior to the crack. The vertical movement of the heel bulb on the affected side further complicates this instability. Causes of quarter cracks may include trauma to the coronary band; preexisting damage to the corium from infection; abnormal hoof conformation - especially the long toe underrun heel; focal foot imbalances; short shoes; or an abnormal landing pattern when the foot strikes the ground."

I see both genetic and environmental factors mentioned as possible causes.