I agree with Paten that, perhaps, they were older than they were led to believe? Or, maybe they were not as healthy as they seemed?
The only other thought that I have is the food. TwitterBug (before there was a Twitter.), our canary, was healthy and happy. One day, we couldn't get his special food that we ordered and had mailed to us. So, we were eventually forced to go to a pet shop and get something temporary. It was meant for canaries, and had okay ingredients, but he died after eating it.
Not long after came the news that pets like dogs were dying from ingredients in their food suddenly. I don't think it was a coincidence.
So, something could have been in their food, maybe? They tell us when there's too much vitamin D, or there's some other problem with dog/cat food. But, when have you ever heard that hamster or canary food has been recalled because it tested positive for some problem or other?
Whatever happened, I don't believe that it was a failing in Dgd's care, and I'm very sorry that it happened.
Love All Life; Thank You For Posting! :)