The vast majority of the pre-COVID related sudden cardiac deaths in athletes would've been arrhythmias - aka problems with the electrical and wiring systems of the heart.

One of the most common causes of exercise-induced death pre-COVID was something called hypertrophic obstructive cardiomyopathy (HOCM), which is a genetic defect where high-performance exercise causes an unnatural thickening of the myocardium in a non-concentric way, eventually causing the outflow portal of the left side of the heart to become so narrowed that an athlete will suddenly pass out and die in a situation when oxygen demand outstrips supply.

So I would suspect that adding in the mRNA spike protein X-factor, the sudden deaths have to do with subclinical inflammation of the heart/subclinical myocarditis that leads to remodeling or scarring of the heart muscle, producing fatal arrhythmias due to wiring disruption or thickening of the heart muscle in such a way that leads to obstruction in athletes with the genetic predisposition to HOCM.