judging from the sudden nature of his passing, i say it would be either a quick battle with a severe illness, a massive heart attack, a major stroke, some sort of accident, suicide (which i doubt that's the case of this shocking untimely death), or he may had cancer for a while but kept it out of the public especially after the passing of Stuart Scott a while back from cancer.
either way, his death was one of those deaths i did not see him coming, and it's pretty sad, he was a great sportscaster too. RIP John, my thoughts and prayers go out to his family, friends, and co-workers at ESPN/ABC/Disney.