Teeth, especially our molar teeth (the back ones) get tremendous amounts of forces exerted onto them as a result of normal day to day chewing and eating. Pair these forces up with say crunchy foods or snacks and the force that is exerted on these teeth increases dramatically. All of these forces can lead to fracture of healthy teeth, let alone teeth with large old silver fillings. Another reason a crown is necessary for a back tooth is following root canal therapy. After the root canals therapy is done, the tooth becomes considerably weaker. This weakening process leads to the tooth being more susceptible to fracture. This is what happened in the picture above. The patient had his molars root canal treated, but because of insurance limitations, no crowns were placed on the teeth. Patient presented 3 months after the root canal completion with a cracked tooth.
Unfortunately not much can be done to salvage the tooth. It needs to be extracted.