The story goes that it cracked from being rung so hard to warn of an impending British attack but how could that be? It appears to be be more than an inch thick and bronze is prettt tough.
According to Wikipedia:
The bell cracked in 1753, the first time it was rung. It was recast twice. The third bell cracked at an undetermined date, and was repaired in February 1846 by stop-drilling. On February 22, 1846 the bell was rung for several hours and the crack extended beyond the repair.
Or what friedo said.
Bells crack more often than you’d think. Usually it’s during casting, due to unequal cooling of the metal. This is called a “failed casting” and the bigger the bell, the more likely it is.
Even after casting, a bell can crack if there’s too much tin in the bronze, air bubbles in the casting, or if too heavy a clapper is installed. Big Ben, a 13 tonner, has a crack caused by a heavy clapper. The crack helps give it its characteristically “foggy” tone.