My guess is that something was wrong with her to weaken her. If she was not in a breeze, 8 degrees fahrenheit should not have bothered her that much in a cosy nest box. If she is not in a breeze, wind chill does not mean anything. It could have been many things so anything I guess is a wild guess. I do suggest checking after dark for roost mites. They live in cracks and only come out at night. In that weather you would think they would be pretty inactive, but I'm not sure. I had a University Ag professor who raised chickens for show tell me that roost mites kill more broodies than anything else, just by weakening them. Her being in a nest may have made her more susceptible.
With her actions, it does sound like she was at the bottom of the pecking order. That makes me think something was wrong with her. It is not your fault.
I'm not saying it is definately roost mites. It could be many different thigns, but I suggest checking as a precaution to protect the others.
I am sorry this happened to you. It is rough.