Trauma - Acute trauma to the jaws such as a car accident, a fall, a punch, etc. The acute pain and loss of function is usually responsive to conservative treatment. Sometimes trauma to the joint can cause chronic damage which may eventually contribute to a TMD problem at a later time. Bruxism - Bruxism refers to a non functional grinding and clenching of the teeth. Some do this while awake but more often it is done while sleeping.
Most people grind their teeth while sleeping to some degree.
For whatever reason some people do this very hard to the point where they wear the enamel from their teeth. This bruxing is done by the jaw muscles and by the morning they can be painful due to fatigue. This constant pressure also can damage the TMJs over time. Sometimes when the teeth do not bite together in harmony with the shape and position of the joints it can place pressure on the joints. Missing teeth can sometimes contribute to this as well.