It occurs due to trauma (getting punched in the face, motor vehicle accident, etc) or from STRESS.
Stress can cause some individuals to clench their teeth together. Clenchers oftentimes don’t know they are clenchers till someone points it out. In fact, they will deny they clench until they are blue in the face. Weeks will go by, and they will all of a sudden realize they clench, and say "OMG, you were right!” (although not usually with such enthusiasm).
Clenching is not great for your teeth, and it is not great for your jaw, head, or neck. When you clench your jaw muscles become tense. They can become very sore as well, and contribute to many problems. Tense jaw muscles can cause jaw pain, neck pain, even headaches.