There are many different versions of how Cliff died, no matter what anyone says, this is the official one as found in "The History of Metallica: A Time Line" in the tab book: "Metallica: Riff by Riff" (authorized edition) produced by Cherry Lane Music . This is the story:

During Ozzy Osbourne's "Ultimate Sin" tour, en route from stockholm to Copenhagen, at about 6:30a.m. on September 27, 1986, Metallica's tour bus went out of control when the driver overcorrected a steering problem. The bus spun around backwards, and the back of the bus started sliding and bumping. Cliff, in the upper bunk at the rear of the bus, woke up and tried to climb out the window. When he was halfway out, the bus slid to the side of the road and rolled over, pinning him underneath.