I am sorry this has happened. Something we can't can't understand in the animal kingdom and this is one for me. Hormones has a lot to do with it and dominance. Drakes are known to gang up on females and drown them and I have seen my geese gang up on a female Muscovy in the pool [Male female geese] so it isn't uncommon. Just no idea why they tend to single out another flock member unless they pick up on something we can't see like sickness or weakness. As for drowning any of the others, that is something you'll have to watch closely for. Is he picking on someone now? That maybe just a one time incident I sure hope so.