Over-reaching occurs when the toe of the hind foot extends forward and strikes the heel, coronary band, fetlock or flexor tendon of the forefoot on the same side. Usually the repeeated 'striking in'will leave cause injury such as bruising or open cuts or sores on the back ofthe leg. It's indication that the horse is moving out of balance, either in the foot specifically or in the entire body. If the medial/lateral (inside/outside) balance of the hoof is off or that from heel to toe, the footfalls and breakover will be altered. When the hind leg is longer than the fore or the stride is extended behind, the back foot may hit the front. This also may occur with "downhill" horses, which are those that are taller at the hip than at the withers. Laziness, tiredness and inexperience may also result in over-reaching. A conscientious farrier wiil evaluate the horse's movement prior and post trimming and termine the most likely cause of the problem and help you understand root cause and find a solution if possible.
Though from what's said it sounds like trimming cycle has been left too long.