It starts with irritation of the trigeminal nerve. You might have a blood vessel pressing on the nerve, damaging the protective coating around it, which is called the myelin sheath.
Certain diseases, like multiple sclerosis, also can injure the myelin sheath. Sometimes a tumor or a tangle of arteries presses on the nerve.
Your trigeminal nerve can also be injured -- perhaps by surgery, an accident, or a stroke.