Yeung Gor surmised that by the time he arrived for the first time at the Tomb of Swords, Dook Goo Kau Bai had likely been dead for around seventy years. I surmise that Dook Goo Kau Bai died in middle age...likely during his fifties. I do not believe that he lived to be a very old man like Cheung 3 Fung.
So what killed him? Given his martial arts supremacy, we can rule out a violent death (unless self-inflicted, but I find this unlikely). Accident is highly unlikely too because accidents happen to clumsy people, but c'mon...this is Dook Goo Kau Bai: it would be lame as hell and utterly implausible that he slipped and broke his neck while getting into a bathtub.
What does that leave us with? Disease...the # 1 killer of people in predmodern times (probably today too, but much more so in the past). I'm ruling out heart attack or stroke because those are ailments that usually result from sitting on one's a$$ all day just pigging out and doing nothing. With his constant martial arts exercise and tremendous inner power (which could clear any blood vessels), dying of any kind of cardio-pulmonary ailment doesn't seem to be in the cards for Dook Goo Kau Bai.
So that leaves us with infectious diseases and cancer. The poll lists some of the likely suspects.