What could’ve caused her death?

[Cloverr39]

I found my 2 year old silkie dead this morning. She was in a nesting box laying next to her sister who is broody. I didn't notice any obvious symptoms. Her beak was wet and had shavings stuck to it. She's always been a weird hen. She's missing scales on her legs and when I first got her she would sometimes have clicking sounds when she breathed when you picked her up. Yesterday when I locked up the coop she was sitting in that nest box with her 2 sisters. One of them is broody and it's not uncommon for one or both to spend the night in the nest box with her instead of roosting.
All I noticed in this hen was that she didn't get down from the roost a couple days ago. When I put her on the ground she wasn't interested in food. For the past month or two she occasionally just stood outside not doing anything. Not super puffed up, but she looked uncomfortable.
I did have coccidiosis in the flock about 2 months ago, but it only affected the younger chicks. All birds in the flock were treated for it though. I also may have a respiratory infection in the flock, but it's only occasional sneezing. It's been there for years.

 

[Eggcessive]

Sorry that you lost your silkie. Had she been eating and drinking normally, or could she been starting to go broody as well. Did she seem to have good weight on her? Was her crop emptying overnight? Did her poops look normal? I usually do a necropsy/autopsy on my chickens that die, to look for reproductive disorders, cancer, crop, or other problems. A vet familiar with chickens or a poultry lab can do a more thorough necropsy. I would check some of those things in your remaining chickens to watch for signs of illness. I always break my broody hens right away if I am not hatching eggs, since they can become run down and weak.