The most common cause of such a situation would be water deprivation followed by over drinking when water was provided.Well perhaps i have missed something in my duck raising journey but NEVER encountered that .... and i have raised many a duckling(and yes ones with no mama), the only reason i would think they would gorge themselves on water would be if they were lacking in it. Ducks MUST have water when they have food, no if's on that.. is it possible this is what happened?