Horses with injuries or diseases of the foot and injuries or soreness of the legs are not striding correctly and the result can be a lost shoe. The health and soundness of your horse is absolutely necessary if he is not going to be stepping on his shoes and ripping them off. Most horses will not put out the effort it takes to adjust their balance to compensate for the weight of a rider. Left to their own devices, they move along with their weight over their front legs, slowing the front feet down, just asking for a back foot to pull off a shoe. It is the rider's job to learn how to help the horse stay balanced.
Circumstances sometimes take ahold.
Any horse can become startled, lazy or distracted and occasionally take a misstep and pull off a shoe by stepping on it. Sometimes another horse will step on his shoe, as in a trailer or crowding under a small tree for shade. Sometimes shoeing the horse correctly can allow the horse to pull his shoes. Leaving the correct amount of expansion in the quarters and length of support in the heels will give the horse something to step on. This is a gamble that many farriers and horse owners are willing to take in order to develop healthy feet and lessen strain on tendons.