Coke overdose?
The friend of a Getbigger who worked with Bob on one of his tours told me he thinks that it's just systemic failure from using drugs - specifically coke.
Bob was fairly open about this, and I even remember an episode of a show running on the Canadian Netflix where he was on the beach / beach chair, where he said he was on Vicodin for something like 50% of the episodes of Full House.
Something like that.
So Bob wasn't exactly private about his drug usage.
So if I had to guess, at 65, assuming he was still using, heart or systemic failure is completely possible.
I had thought Bob had put the drugs away in recent years, but from what my friend [who is friends with a Getbigger] is saying, that's not true.