Throughout the horse's life, its teeth are pushed upward from the lower jaw and downward from the upper as the surface of the crown within the mouth is worn away. This wearing away takes place as the horse chews. When the horse chews, its lower jaw, which is somewhat narrower than the upper, moves back and forth laterally, grinding the food as lower teeth come into contact with the uppers. As long as there is full lateral movement or excursion of the jaw, normal teeth will wear off evenly. When there isn't full excursion, they tend to wear unevenly, and that can produce problems.
Many dental problems faced by horses essentially are man-made through genetics.
We often breed horses which have dental problems and thus pass those problems on to future generations. If a horse in the wild couldn't crop and chew grass properly, it eventually became weak from starvation and either died or became a victim to a predator. In a domestic setting, there is no quick death verdict for a horse with bad teeth. Dental problems can be coped with and the horse can perform and reproduce throughout a relatively long life. In today's world, horses which are born with normal teeth can develop dental problems because of diet and frequency of feeding.