Definitely not old age Mak ...... they only about 15 weeks old!
And not with a wound on it's back - a fox would have gone for the throat I think, and I'd have thought stoats etc too as they are lower to the ground than the duck. Any dogs around? That's how my old GWP killed one of mine (by accident) - caught it as it was trying to run away from her and grabbed - didn't mean to kill her but Hester had big teeth

Allez used to catch them too but he always picked them up round the body and bring them to hand, as they are supposed to., and they didn't seem to run from him.