Roots, wire fences, rocks and the like are common causes. A horse can pull a shoe this way while you are riding, and also when turned out. If you continually find shoes in the same place in a pasture, there is likely something near by that the horses are catching the shoes on and ripping them off. Remove that object and the problem will go away. If this continually happens when riding in a certain area, perhaps you shouldn't ride there, or be more careful about picking your route.
Forging is a precursor to loosing a shoe.
Horses forge when the hind shoe comes up and hits the bottom of the front foot. Forging is a gait timing abnormality. It is caused by the front feet staying on the ground too long, thus allowing the hinds to catch up. It can be caused by many factors. This is actually a rider problem.