You might have just found your answer without even knowing it. The compost pile. How old is it? Mold in it? Botulism perhaps?
My other thinking is since you're only a week from slaughter, I wouldn't treat no matter what is it. Coccidiosis which was one of my first thoughts when I read the original post, only going to be in the digestive system, and since you're not eating that part, and you can't eat them right after treating them, then I say GOOD LUCK with the rest, and just hope and pray nothing else shows up. (That's gotta be the longest run-on sentence ever! Ugh!)
Very well could be just a fluke that we might not ever know why it died. I'm doubting it's contagious though. At least not in a way that will affect the week away slaughter, and not going to affect the meat of the rest.
If can try contacting the state vet to do a necropsy but by the time you get an answer to what happened, you'll have the others in the freezer anyway. Of course for peace of mind, they are typically free, or low cost in most states. Your local extension office can get you in contact with the dept you need, if that's a route you want to take.
Did you by chance cut open the bird to see if you noticed anything unusual? That also is a great learning tool. I still remember most of the chicken anatomy from processing 49 roosters one 4th of July many, many years ago! I think I was around 12-13 yrs old!