Marland likely found Agnes infuriating to work with. We know how involved he liked to be in the writing of his shows, and to have to answer or collaborate with someone like Nixon must have been hard for him.
Patrick Mulcahey, who wrote for Marland at both GL and Loving, once mentioned how much he disliked Nixon's involvement in Loving's writing process:
How did you start in Santa Barbara ?
Thereby hangs a tale. After working with Douglas Marland on Guiding Light and then on Loving, which I hated (and where Agnes Nixon was like some psychotic schoolmarm on speed, making copious condescending red-pen "corrections" in the margins of scripts - "You used the same word on page 2 and on page 34 ! Too repetitive !") - after that, I decided I was done with writing for soaps. Douglas was the best. He'd taught me more about writing than any ten literature professors ever could have, plus I'd won an Emmy. I figured I'd never have another experience like that, so I decided go back to what I knew best : waiting on tables and writing plays at night and being a starving artist again.