A lot of people don't take heat in consideration when working out their pacing strategy. They may have been able to hold PB pace on a cold spring day but forget the effect that the heat will have so go out too fast for the conditions.
I'd go with the, lack of conditioning / pushing too hard theory.
I did a race in August where there were distances of 11, 20, 32 or 64 miles. It was really hot on the day but I only saw 1 casualty out of around 600 runners. Probay due to the fact it only attracted experienced runners.