Cigars crack mostly because the wrapper leaf has dried out. Some cigar wrappers like the Cameroon wrapper are very thin and fragile and can dry out quickly if not stored correctly. Cigars can also crack if they are crushed or compressed on the sides like if you squeeze one.
If you have a cigar with a crack near the foot, you can cut the cracked section off so you can still smoke it. If the crack is in the middle or close to the cigar cap, then it is un-smokable.
Here is a cracked cigar where the crack ruined the cigar and now cannot be smoked. I had it replaced by the cigar retailer.