Was she a hybrid production breed? They really aren't meant to last. We raised 3 brown shaver from chicks and they've all passed away this last winter at 3 years old. The weather here is quite mild, no freezing temps - two went on hunger strikes and the third had acites and suspicious abdominal lumps. Seemed to hit each of them post-moult, when they should have been getting back on the lay. Definitely wasn't anything infectious or worms; hit them each individually months apart too. Our first was a hunger strike and we caught it quite late... The last death was also a hunger strike, but we caught it right away and brought her in for medical attention. Unfortunately, it doesn't appear to have been worms or cocci and she was showing signs of organ failure, so we made the call to cull after a week of slow decline with treatment.
Bad genetics could also be a factor. We lost a heritage pullet to a raft of things when she was almost one year old. The breeder was breeding father daughter, but had only just established her breeding flock and I don't think she started with diverse genetics, so I'd already suspected inbreeding after losing a pair of chicks one late failure pre-hatch and one post hatch to 'mysterious' joint issues.