Its probably just coincidence. I have a lot of ex battery hens running free range. Some I got 4 years ago, are just dying off now, which is a good life span. But on the other hand, I have noticed that some of them go very quick with no warning, and then another will follow.
Thinking about it, they lay and lay in the farms, so assuming they are 15 months when they come out, plus another year, thats probably about right, given their workload.