Unclear. Different reasons: viral, inflammatory, and most of the time it's not clear.
Couple things. It is not always known but the known things are trauma to the trigeminal nerve and compression of the nerve as it exits the scull.
TN causes. The most common form of paroxysmal orofacial pain is TN. When a demonstrable structural lesion such as a tumor (e.g., meningioma), vascular malformation or demyelinating disease (multiple sclerosis) has been identified, TN is classified as symptomatic. When the cause of TN is unknown, it is classified as idiopathic. See an orofacial pain specialist for diagnosis and management.