"death throes" would explain that. Having never processed a duck I don't have personal experience with them there, but I have done my fair share of chickens. Some birds will really flap and kick, some only a little. I have learned during my processing to always have a firm grip to hold them in their cone just in case... I had one bird (one of my first) flap and kick its way out and onto the ground... As cruel as it sounds (I'm sorry for this by the way, I don't want to rub salt in wounds) have you ever heard the saying "running around like a chicken with its head cut off"? Could explain the filthy state you found him in too.
