I feel terrible asking this, but what caused this fatality and, more importantly, how could it have been avoided? I think I never actually considered the possibility of something like this happening on the mat until now.
There have actually been several training deaths in Japan. To my knowledge, each was in a University club setting, no Shihan was presiding, and basically the issue was what we would call "hazing". The upper class seniors presiding over the class had the juniors training to total exhaustion. Things like 1000 break falls from shihonage. The persons killed ended up being to tired to protect themselves and took disastrous falls.
There's no question that this kind of thing is avoidable. But it involves a change in the dominant paradigm operating in these university clubs.
It's not that there aren't Shihan who are brutal. They injure folks all the time. It's just that their skill level allows them to keep a lid on it. When someone is really in trouble, they can usually find a way to avoid this kind of over the top injury. But some Nidan college kid doesn't necessarily have the sensitivity, the judgment, or even the developed sense of the consequences of his actions to be given that kind of power over his juniors. It's a recipe for disaster.
Aikido Journal Article About Injuries in Aikido