I had my first horse boarded for a few months, and he was turned out with the "big herd" every morning. He was at the bottom of the food chain, and he constantly came in with random little hairless patches on him where he was kicked at and the aggressor's hooves removed the hair without breaking the skin. Never had a problem when he came home and was an only horse or turned out with one other gelding. If she's running with other horses, this may be what's happening, and it's kind of an occupational hazard of turnout. I wouldn't discount the fences, though.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown