Why so many deaths in triathlons... nearly all in the swimming phase?

Panic attacks. Officially, drowning, but the root cause may be panic:

“My world is anatomic,” said Steven Shapiro, Vermont’s chief medical examiner, whose office investigated the deaths of two men this year. “I can’t point to the body and say: ‘There’s the panic attack, there’s the arrhythmia.’ Once you’re in my office, there’s no panic and the rhythm is asystole,” he said, using the medical term for cardiac standstill.
Nevertheless, circumstantial evidence strongly points to panic, with the biggest piece of evidence the most obvious. Something is happening in the swim that isn’t happening on the bike or run.

“I was taken by surprise the very first time I did a triathlon,” said James A. Millward, a 50-year-old history professor at Georgetown University. “I swam about 50 yards, I couldn’t get into a breathing rhythm, I felt more and more anxious, and I thought, ‘Wow, I’m having a panic attack.’ ”

Millward has done the Nation’s Triathlon four times and has had the sensation each time after jumping into the Potomac River with scores of other racers. The overwhelming urge is to get his head out of the water. 

It's easy to see why the human body may have evolved this response.