Are you sure they're eating them, or just moving them? Look to see if they're spitting them out in a hidey-hole. The parents often "Bring the kids back inside" when they go out too far. Otherwise, if they're actually eating the fry, it's because they feel threatened by something. They eat them to prevent something else from eating them. Yeah... doesn't really make sense to me except maybe they want the calories to create energy to fight back and have more babies while preventing an enemy from caloric energy.... I don't know.