My sister was a trainer at USF when it happened over there last year.  Freaked her out pretty bad.  He came in saying he didn't feel right, and then he was just gone.  Maybe it is the increased training these guys go through now, or maybe you just didn't used to hear about it, but it seems like this type of condition is happening more frequently every year.

The said

It sounds like that heart defect.  Somewhere along the lines of having an enlarged heart.  There was an article about it in SI a few months back (I think it had Favre on the cover).  Very sad and scary.

Typically in young otherwise healthy individuals who die suddenly while exercising it is because of enlarged heart. The ventricles which do the main pumping of the blood have their muscular walls become soo enlarged that the amount of blood circulated can become much smaller than normal.  Add dehydration and extreme physical exertion and it does not mix well.
http://www.emedicine.com/med/topic290.htm  If you want.

I see this has been posted for a long time, but I've been trapped in the hospital so I apologize for the delay.