I know the answer.

A few days after Doobie passed, Betty mentioned something about guttural pouches, which I didn't know what it was, but I still wasn't in the mood to talk about his death, so I didn't ask what it was. It's what she thinks killed him and now that I'm strong enough to investigate, I see that she is in fact, very correct. It was not a stroke at all, but Rupture of the Longus Capitis Muscle. I feel relieved now that I know.