The most common cause of sudden cardiac death in young athletes is hypertrophic obstructive cardiomyopathy (HOCM) which accounts for approximately one-third of cases. The second leading cause is an anomalous origin of a coronary artery causing ischemia. Other causes are listed below:

Myocarditis/dilated cardiomyopathy
Mitral valve prolapse (controversial)
Aortic rupture
Myocardial bridging (controversial)
Aortic stenosis
Arrhythmogenic right ventricular dysplasia (ARVD)
Ischemic heart disease (atherosclerosis)
Undiagnosed congenital heart disease
Genetic channelopathies (i.e. prolonged QT syndrome)