How can a pro athlete die from a cardiac arrest?

Rare, thankfully: Pro athletes who die have an unrecognized condition. The most common, accounting for almost 50% is hypertrophic cardiomyopathy. Dilated cardiomyopathy, congenital coronary anomaly, long qt syndrome, brugada's syndrome, arhythmogenic right ventricular dysplasia, and marfan's aortic dissection are other very rare causes. The incidence is 0.75 per 100, 000 - very rare!

Cardiomyopathy: Cardiomyopathy is either linked or acquired and represents a high risk condition to trigger dangerous arrhythmias during high stress situations. Many times this predisposition can be discovered ahead of time with the appropriate screening ( history, physical exam, and ecg).

Many reasons...: Sometimes there is an unknown congenital heart defect which goes undiagnosed and then results in a sudden cardiac arrest during exercise. Sometimes, an object, like a baseball, hits the chest at just the right time during the cardiac cycle resulting in cardiac arrest. So, there are many reasons for this unfortunate event to occur.