No doubt though, the death zone could kill people playing ping pong. Yet the down-climbing problem is global to mountains of any size. The extreme altitude on Everest amplifies it. More accidents occur on the way down on mountains less than half the size of Everest. The mountaineering bible, "Freedom of the Hills" discusses the phenomenon extensively:
http://www.amazon.com/Mountaineering-Freedom-Hills-Mountaine...
It's a common problem among mountaineers that once the "goal" has been reached they rush home and make careless mistakes. They rush down "like a horse to a barn"
I've seen this problems in many different expedition sports. Once the perceived danger is past, people let their guard down and get hurt. More often than not people train for the hardest part of an expedition. They put so much effort into getting through that part, that when it's over their brain says, "I've done the hard part, the rest is a foregone conclusion." They forget that the uncertainty of their goal made them fearful which in turn made them concentrate. Once people feel more certain, the fear subsides and takes their concentration with it.
On the way down, your eyes are much further away from your steps and the slope is angled away from you. Each step is longer, more dynamic and more likely to dislodge some rock / ice / snow. You are also putting a lot more force on each step so your legs can actually get more tired on the way down. When you fall you mostly go to your butt, slide and you need to roll over on your belly to self arrest. By that time you have often picked up so much speed that it's too late.
All these physical factors plus many well documented "horse to barn" psychological factors make down climbing the most dangerous part of the day.