Myocardial infarction caused by unexplained spontaneous myocarditis? Nothing to do with the vaccine, of course.
Perhaps history of coronary artery disease led to a myocardial infarction (heart attack) and ventricular fibrillation with the immediate cause being an occlusive coronary thrombosis in the left anterior descending artery resulting from a ruptured cholesterol plaque.
It’s fairly common as far as overall longterm wellness is concerned, usually because of poor health choices.