When you have an unexplained death, it really is a good thing to get a necropsy so you know what you're dealing with. You might be able to prevent further deaths if you know what killed her. But you need to have it done very soon after death since decomposition begins quickly. If you haven't disposed of the corpse yet, refrigerate it until you can locate where to send it.
Another thing you need to do is to observe your flock to see what they're doing to each other. There are many causes of feather loss. Making assumptions doesn't do anything to identify the problem. You need to identify the problem to solve it.
If feather picking is the cause for the loss of feathers on the backs, you should be able to see who's doing it. A lot of times, it's a bully who is picking on someone lower in the pecking order. Sometimes you can segregate the victim for a while by putting her in an enclosure adjacent to the others, giving her time to heal and regrow her feathers while gaining more self confidence to stand up to the bully.
But until you observe the flock, you really don't know what's happening, so that's the first thing you need to do.