My guess is a congenital heart condition that went undetected.
That happened at our church's day school years ago. During gym class, a second grader dropped dead. She would be in her early thirties now. My niece was in the class. It was hard on the family, of course, and hard on the teacher too.
Most likely a condition called athletic heart. Difficult to detect without a test called an echocardiogram. Regular exam in a drs office will not pick it up
This sometimes happens with new recruits (Army) whose condition is not discovered until they have a heart attack during PT. Tragic.