Nash's non-alcoholic fatty liver disease. Happens to humans who are overweight. In humans its caused by poor diet and exercise. High cholesterol, high fat, high protein, these just to name a few.
Pretty much the liver has too much fat around it doesn't work properly and can lead to a lot of other issues. Liver like most organs is very important, the list of illnesses that could come from this is long.
Pretty much you got to exercise and feed your waterfowl not So Much Junk Food it looks like? It's tough for me to diagnose because I was under the impression that these feeds we're pretty decent and not so much junk in it!
How much "junk food' do they get? What was her weight compared to the weight that she should be?
This was definitely something to go overover your vet on how she was able to become obese.
Alternatively she could have be diabetic, not sure if chickens can become diabetic but that is an aftermath of fatty liver disease as well.
Hope this was able to help a little bit definitely do some research yourself it's something you should know as a human as well! And definitely go over it with your vet!!