Oh it must have been his diet. /s
It could have been his diet. A lot of fitness types are religious about diet, but, unfortunately, often listen to the wrong people, and often fall for conventional wisdom myths. I know a lot of very fit people whose diet has far too little healthy fat, far too much in the way of carbohydrates (especially fruits), and too much protein.
Also, it could have been the exercise itself. Exercise is very damaging to your body. In small amounts, with plenty of rest in between bouts of exercise, the benefits can outweigh the costs. But a lot of fitness addicts go well beyond that line. And the organ most damaged? The heart (at least, according to a very good cardiologist I know personally).