2008 was a bad year for bats. In just three months of the Major League Baseball season, 2,280 broke and of those, 756 shattered into multiple pieces, which can be dangerous for players and spectators who could be impaled by the debris.
Blame maple. At high densities, maple is stronger than ash, but at lower densities, it is weak, especially when it collides with a high-velocity object, like a baseball. Another factor is the slope of grain, the straightness of the wood grain along the length of the bat. If it's not parallel, the bat can lose strength.
New restrictions and manufacturing processes have improved bat strength; the number of shattered bats has fallen by half in the past five years.
Sources: Major League Baseball, U.S. Forest Service