IMO, it sounds like he was messed around with at a young age by chickens and now that he's older and bigger he might be looking for revenge? I don't know, but when I had my geese I had this one duckling that always and I mean always pestered them and once the geese and were bigger the table turned one day. If you honestly think its the muscovy drake then I would seriously consider cooping him up and see if it keeps happening. If it's him then I would just butcher him and eat him.