Bad Duck Smell Question

Customer brought me three birds a few weeks ago. He lives less than five minutes from the WMA where he killed them and called when he killed them and when he put them in the freezer. Yeah he calls a lot when he kills ducks he wants to mount lol. Anyway they were killed and frozen within 2 hours. 2 pintails and a redhead. I caped them out last week and the hen pintail and the drake redhead both smelled rotten. Like gag a maggot kind of smell. Upwards of the vent, in the abdomen, the inside of the skin was a greenish color. Neither bird had shots on the abdomen and neither had feather loss or slippage. I thought maybe since they were killed next to the beach this could be the cause but I have done salty divers before that smelled like fish and this put that to shame. Pintail drake was normal and all mounted up just fine. What would cause this? Thanks in advance for any similar experiences or possible explanations.