The Mitsubishi MU-2B has a very long & detailed SFAR 108 that details the trained and currency requirements for anyone who wishes to manipulate the controls of the plane. I'm wondering what incidents or accidents and unusual handling characteristics required a Special SFR for this plane.
Since the MU-2 weighs less than 12,500 lb, a type rating is not required to fly the plane, so pilots only need a multi-engine rating. However, it is a high performance pressurized aircraft that requires more skill to fly. Mitsubishi Heavy Industries asked the FAA to create a type rating requirement for the MU-2, but the FAA decided on creating the SFAR instead, which is more strict than a type rating, especially in the amount of training specifically required.
Research showed that the accident rate of the MU-2 from 2001 to 2005 was not really exceptionally high compared to aircraft of similar size and performance. While the accident rate was higher than some, it was also lower than others. However, the FAA received a lot of pressure to create a rule for the MU-2.
Regardless, the accident rate has gone down significantly since the FAA created new regulations for the plane.
According to Wikipedia (or at least my reading of the article) http://en.wikipedia.org/wiki/Mitsubishi_MU-2
It looks like there were a lot of incidents in the plane, enough that the FAA seems to have investigated the plane specifically and now advises/requires some special training for it. As any government agency would they also issued lots of paperwork as a result thus your findings.
The MU-2 has had a very bad safety record. From various sources I've talked with over the years, if you lose an engine, you need an excessive amount of altitude to recover. Various design issues I've heard are related to it, from possibly overpowered engines, to short wings. All these things coupled basically put the FAA to put in a Special rules for anyone wanting to fly the MU2. Same with the other SFARs, there's usually been a rash of incidents outside of what's normally taught for the category/class, that requires special or additional training.