There’s studies on student cardiac arrest that long predate Covid, you can find them on PubMed. Student athletes have a significantly higher rate of cardiac events, including death, than their non athlete age cohort. There’s been no identifiable common explanation, but the higher incidence of cardiac issues has been known for years. Maybe the stress of training and high level performance triggers latent problems that otherwise would remain dormant.
You can have cardiac anomalies that only manifest in youth via
Athletics
Heat
Altitude
Some drugs
Speaking from experience I knew by late teens something was amiss