There are a number of causes for cracked teeth.. The most common cause is they have overly large fillings . When a tooth has a large filling , it can have its tooth structure 'compromised' and weakened. Teeth that have broad fillings place or have three or more surfaces filled are in what I call the 'crown zone'. They should be protected by full coverage(crowning). I try not to place overly large 'permanent' fillings in teeth at all but instead recommend crowns. Crowns cover the entire biting surface and redistribute the vertical forces on teeth so that the tooth is less likely to fracture.
Another possible cause of cracked teeth is tooth grinding or bruxing at night. Some patients give their teeth a real workout at night and these patients have more cracked teeth than others. They should wear night guards to protect their teeth while they are sleeping.
Using your teeth to crack Ice cubes, bones or chewing on a pipe stem, can crack teeth. Teeth, although hard, can with repeated stresses tend to develop cracks.
Still another cause of cracked or broken teeth are 'food accidents'. Patients eating something soft such as chicken salad, tend to bite hard on these easy to chew foods. If they unexpectedly bite down on something hard (like a chicken bone), they often crack a tooth.