I wasn't there when it happened, but I believe it was cancer...
Here's a list of how all my cats have died, in the order we got them:
Baba and Teddie (twin Tortoiseshells)
Baba: Age-related illness, had to be put down because she was having seizures; she was like, 19/20.
Teddie: Cancer, put down, I believe she was 16 or so.
Dogger (I named him, I was like, seven; I think he was black with white markings): Run over
Ash and Lily (brother and sister cats; Ash, the boy, was a tabby while Lily, the girl, was a predominantly white Tortoiseshell)
Ash: Run over, presumably while looking for his sister who had died about a month prior.
Lily: Run over
Bobby: Still alive
Pippin: A stray we adopted, never truly settled with us and disappeared during Christmas 2017, he was a tabby who looked a lot like
the last cat in this meme (just much, much thinner)
Dogger was the first to die; Teddie died several years later, and Baba died a few years after that. Then we got Ash and Lily, and after they died we got Bobby, the cat we have now.