In 51% of such cases in military forces, causes are cardiomyopathy, cardiac rhythm disturbances, hypertensive heart disease, congestive heart failure.
It could have been a severe heart attack where some plaque ruptured, blocked the lower end of the vessel, and a clot formed at the rupture site, causing a double whammy.
Chances are that your father suffered a cardiac arrest due to some type of arrhythmia or irregular heart beat. This is NOT heart failure which is a chronic form of heart disease where the muscles of the heart are diseased. Arrhythmias can cause a sudden death event and many young athelets have died on the basketballcourts or on the football field due to arrhythmias. your father needs to be evaluated by an EP Specialist who deals with the electrical issues with the heart.