I accidentally became a non-fiction ghostwriter in the day job when a very big computer network took me on as Documentation Officer, with the task of editing and publishing technical material written by technical experts. As the experts were all, by definition, very busy and had no time to put pen to paper, I soon worked out that everyone was happier if I simply interviewed them, then wrote up their material on their behalf.
Meanwhile a fiction editor with whom I had previously worked changed jobs and inherited a series that required a ghostwriter. She thought of me. I still kept the day job for a while longer, until finally in 2015 the ghostwriting developed into enough work that I could chuck in the day job and go full time.
And the rest is history.