It would make for better copy to say that Shadow and Cleo were like some couple that had been married for 70 years or so, and one just couldn't live without each other. It happens to human couples, and probably to animal couples as well. But except for Shadow's final year, as she started wearing down, she and Cleo rarely interacted, and usually negatively when they did cross paths; both in fact were loners as far as other cats were concerned, although both were also very needy when it came to people.
More practically, if one needs a proximal cause other than the basic fact that Cleo was very old, we went out of town for a few days right after the 4th of July, and returned to a fairly nasty slab leak. While everything's fixable, we have to redo the kitchen and one of the bedrooms, just drywall and flooring, but disruptive enough to where the cats couldn't come into the house for a couple weeks while we were tearing out walls and floors and running dehydrators. (We have a covered, locked porch where they sleep at night regardless, so these are not outdoor cats.)
Anyhoo, spending the better part of a month straight out back, with strange machines running in the house, people coming and going, etc., might just have been disruptive enough to give poor Cleo a shove. I really don't know. I know that when we found her eighteen years ago almost to the day, she was a scrawny, starving kitten, days or maybe hours away from death. Chances are if we hadn't found her when we did, she would wandered back out to the field, to die of thirst or be grabbed by one of the hawks that cruise through regularly looking for unwary mice.
So you do what you can, and they get a pretty good run, our animal friends. But we miss them all the same.