So far cat deaths are an epidemic in dwarf fortress and from what I've seen in the combat logs there are no prior report to a cat death, meaning their death aren't tied to any monster attacks.
Based on my observations in my fort, cats often carry the syndrome dizziness with them now, and their death always leave a pool of vomit with them. These are synonymous with the effects of alcohol poisoning, but I did not have my suspicions confirmed until I stumbled upon this log:
As you can see, the cat is covered in vomit and dwarven wine
My working theory is that there's a bug with taverns right now that lets them serve alcohol to animals that are present in the tavern, which I confirmed with the following screenshots:
This has happened for every animal inside the tavern, with the odd exception of this goose
EDIT: DeKaFu has found the Issue: Tavern floors are often covered in alcohol due to splashes, which then cover everyone in the tavern. Cats have a self cleaning interaction that also lets them ingest syndrome causing substances, like forgotten beast goo. This coincidentally includes alcohol and the effects of inebriation. This means cats covered in alcohol will clean themselves and get drunk, due their small size and the [SIZE_DILUTES] tag for alcoholic beverages, they're very likely to die from alcohol poisoning. A working solution right now is to keep them away from taverns. This could be done by pasturing them, which unfortunately takes away their invaluable contribution to vermin control.