Most likely another chicken is plucking her rear feathers out. This can stem from things like boredom (coop too small, etc.) and nutrient deficiency (low protein feed, etc.).
I think sand on concrete is a recipe for stinky bacteria buildup. I'd suggest a very absorbent bedding (wood pellets, shavings, hemp) perhaps over vinyl sheet flooring.
Sorry for your loss. It sounds like she was molting and it could have had to do with that. But if you didn't see any external injuries or other abnormalities, it's very hard to tell without doing a necropsy.
He's gorgeous. I have an affinity for white chickens.
I have a question - How do you keep your roosters from getting major frostbite in the winter? It's my first winter with a rooster and he has a large single comb.
If your coop doesn't have a dirt floor you're better off doing the deep bedding method (no moisture) and keeping it dry. Dry poop = fewer pests and a lot less mess. In the run, the litter will break down into the ground eventually, but it may be slower if you have a roof on the run or the...
How wet? If it's just damp when it rains or snows and they have a large, very dry coop to retreat to I wouldn't worry about it. If it's a swamp then you need to do something.