From what I've been able to gather, it's a combination of susceptibility (pre-disposition to heart attack, stroke, etc.), dehydration, and sedentation (I just made that up; means sitting for long periods of time), and one more element...
What wraps it up in this case is the perceived
danger that you get regularly when playing FPS's, MUD's, MORPG's causes stresses that you internalize because even though your eyes tell your brain that you're being shot at, chased, and lit on fire, you know
it's not real... but your subconscious doesn't know that.
So take somebody prone to kick early, distract him so forget he needs water to survive, let him sit in a chair for hours on end, and then terrify him subliminally
and *snap*... dead customer.
It's a theory, anyways...