I wonder what the number of deaths are in pure open water swim events. Many have the same mass starts and are usually more than 1 mile. I know of several open water swim deaths from the past year or two, but they involved very long solo swims, very hot, or very cold water. By nature these races are mostly made up of athletes with swimming as their main sport.
I see plenty of people at tri-races that are putting on their wetsuits for the very first time, swimming in a lake or ocean for the first time, and really did very little swim training. The mental anxiety of swimming in dark water cannot be ignored. I have seen plenty of above average pool swimmers completely freak out in lake and ocean swims. I would think that this anxiety on top of the pre-race anxiety, adrenaline, and the chaos of the swim start would get the heart racing.
I tell anyone that is thinking of getting into triathlon to not underestimate the swim. I try to get them into our masters program and out for at least 2 ocean/lake swims. Sadly most non-swimmer first time racers think of the swim leg like it is a swim at the pond on the 4th of July.
No matter the cause, these are tragic events. My thoughts and prayers to him and his family. Be safe out there!