That is a hard one. When you deal with living animals you sometimes have to deal with dead animals. That does not make it any easier though.
I've found a few dead like that. Sometimes I have a clue of what might have happened, but often there is no indication whatever.
Was it anything you did? Probably not. Sometimes chickens with something wrong with them just die, say like a weak heart finally just gives out. Something like a heart attack or stroke. It is unlikely but possible she found something poisonous to eat. I hesitate to even include that one since it is so rare and ususally if they eat something poisonous, they don't act right for a while. Sometimes, if they get too much calcium when chicks they can have longterm kidney damage which can eventually kill them, but again they usually don't act right when their kidneys shut down. They don't die instantly. Same with an egg bound hen. Since you are free ranging them it is unlikely you are overfeeding them to the point that they get fatty liver syndrome and die. Like most things that could remotely be considered your fault, they usually don't act right for a while before they die with fatty liver syndrome.
So with her just dropping dead like that, I'd say it is highly unlikely it is anything you did.
