The cause of SADS is very often a congenital heart defect. The high heart rate and blood pressure associated with endurance efforts is a good way of triggering an underlying defect that may be benign in a relaxed state.

But to state that these people died because they "underprepared" is ridiculous. That's like saying you could have prevented dying from a brain tumor by doing more crossword puzzles. Just as a tumor, a congenital defect was just a bad dice throw. Exercise triggered it, but so could running to catch a bus.