How did author Pendragon become king?

Uther Pendragon - King Arthur's father - is known only from the Arthur story. The Arthur story never mentions how Uther becomes king.

pendragon is probably a military title, rather than a name. We find it also used of the Heroic Age leader LLywarch Hen:

pen dragon, pen dreic oedd arnaw

(he had the head of a general, the head of a dragon)