Sorry for your loss.
As others have mentioned, these battery hens were most likely bred for maximum egg output at the expense of a shorter life. The other day I was reading some background on life expectancy of my chicken breeds in my backyard flock. My hybrid egg production hen has an expected life of 2-3 years, whereas some of the heritage birds can live 5-6 years. So it's not just battery hens with short lives, it seems a number of hybrids have a short life.
I think you should be glad that you were able to provide some love and support for these former battery hens. Chickens have short lives, so I would suggest just enjoying them and caring for them the best you can do. You probably have done everything right and your birds are probably otherwise healthy (since you did not mention any other concerns). Focus your attention on the other girls and maybe plan for replacements if you plan on keeping chickens for longer. Best wishes.