It occurs due to trauma (getting hit in the face, motor vehicle accident, taking a bite out of something too large, etc), or from STRESS.
Stress can cause some individuals to clench their teeth together. Clenchers oftentimes don’t know they are clenching until 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.
If you wake up with headaches or jaw pain, you are likely clenching at night.
TMJ (temporomandibular joint) problems can lead to clicking and popping. If severe enough, the jaw can sometimes lock. Depending on how it is locked, the jaw will not fully open, or fully close.
Stop letting TMJ ruin your day.