Manufacturers position each bat's label over the mechanically weaker side of the wood. To reduce chance of fracture, and maybe deliver more energy to the ball, a bat is intended to be held so the label faces sky or ground when it strikes the ball during a horizontal swing. In this orientation, the bat is considered stiffer and less likely to break.
Maple bats in particular were once known (circa 2008) to potentially shatter in a way that resulted in many sharp edges, sometimes creating more dangerous projectiles when a bat broke. Maple bat manufacture evolved significantly, in cooperation with Major League Baseball, paying special attention to grain slope, and including an ink spot test to confirm safest wood grain orientation.