The theory used to be that a dog ate poop because it craved something that it wasn't getting in it's diet.
The newest theory is that it's a maternal instinct. Mama dogs will eat their puppies poop to keep the area clean and remove odors that might attract predators. The puppies will observe mama and do it too, but most pups stop by the time they are weaned. Evidently, yours is reliving her childhood.
