For me initially it was a sense of urgency, age was creeping up, there was a story to tell, and unless I wrote it, all would be lost.
Then I discovered an old manuscript written by my grandmother that never made it to publication. It was much too valuable to ignore. So I published that, in her name of course.
Once I made these small incursions into the art of writing, others seemed to follow as a matter of course. I enjoy picking up a topic, teasing my mind with it, researching it, and finally putting it into print.
Surprisingly (to me) it seems to appeal, not just to a small circle, but to a wider market. I joined a class with a fine tutor, and now try to put someting of interest out with fair frequency.