I see well he may of been a bit to 'in love' with her... With ducks it's better to have several ducks per drakes vs pairs, i mean i have a few birds that pair off but the flocks free range so are capable of leaving a demanding male.
Everyone has pretty much given you all the suggestions i have for it, generally Pekin have very white and very waterproof feathers. What kind of set up did they come from? were there many males?? Because of the condition of the other birds i tend to think it's not diet, or parasites(of coarse...
It is more visable on white birds.. down by the tail basically where it joins to the body, you'll often see ducks rubbing their heads and necks along it...