Help, can any one help me figure out how my chicken died?

I have just got back from the plot having found one of my chickens had died. Though i am a little lost as to what is the cause.

she was kept with 6 others in an eggloo cube, in a run 10m x 10m, She was just over a year old. They all have access to layers pellets and fresh water, as well as getting a handfull of mix corn every day.

She had lost a couple of feathers about 10 days ago and had a little bald patch on her right breast about the size of 10pence coin. But there were new feathers growing through so i just sprayed her with this purple spray in case she or the others in the folk were pulling them out though i have never seen this happen.  She also after losing her feathers was keeping a distance from the other hens and looking a little mopey but was still eating and drinking normally and egg laying has remained the same.

There was no sign of red mites but i did have a problem with red mite about 7 months ago but i disinfected there coop and moved them into the new eglu cube and throughly dusted with mite poweder and havent had a problem since then.

apart from her behaviour and the little bit of feather loss there were no obvious signs anything was wrong.  All the others seem perfectly fine but i am at a complete loss as to why this one has died. If any one has any ideas as to what the cause could possibly be i would very much appreciate it.

thank you for your time

Paul

Looks like she wasn't "quite right" before hand. The only way you can tell without clear symptoms is a post-mortem performed by a vet.
The best thing you can do is a good deep clean and disinfect. Have a good look around the run for anything obvious.
Keep a sharp eye out for any symptoms in your other hens.