I have always found this strange, after just two years, given how dedicated - devoted! - she was to Frank Zappa's music and how Zappa himself seemed proud and appreciative of her virtuosity and commitment. Her own husband of then Ian Underwood had quit a little over a year before that with the words "Seven years with the Mothers of Invention is enough for anybody". George Duke also left that super line-up, a little later than Ruth, but like Ian U. he had also been with Zappa for a good while and we know that he was ready by then to pursue a solo (and duo with Stanley Clarke) career in soul-fusion. He was already from the jazz world before joining Zappa, and sometimes "in between", so we guess that at that point he was missing that world and was becoming tired of the RnR world. Ruth Underwood would later do several studio sessions for Frank, and very near his demise, even got invited to do percussion sampling work too, at the Zappas'. So they hadn't fallen out either. It seems none of the books I've read ever mentionned anything about this departure, of consequence to Zappa's music of this era.