Mine does this from time to time, she will refuse to eat her food and then go outside and eat poop. drives me nuts! I think that when she wasnt getting enough nutrients from her food because of the pancreatic exocrine deficiency is what started this off. You might want to get that checked out its somewhat common in GSD’s.
http://en.wikipedia.org/wiki/Exocrine_pancreatic_insufficiency
My vet said that poop is “kind of like cheese” to dogs, it has interesting smells and flavors. You can try some of the food additives that are going to make the poop tast bitter, none of them worked in our case. If you have multiple dogs this can turn into a social behavior between them.