This may not be the typical "young bodybuilder passes away" story.
He was in a bad car accident and bedridden for a while. Suffered broken vertebrae, torn pec, and other broken bones. 12 weeks of recovery and not in the gym. Returns to the gym and collapses the following day.
It very likely could have been a blood clot that developed while bedridden. Which often happens whenever a surgery is followed by a long recovery. The clot ends up lodging in his heart, lungs or a major artery once he resumed activity.