Charlotte left at the beginning of the 8th season because she felt her character had done everything she could do, plus she wanted to move on on other projects. Another reason, maybe not exact, was that DS had been canceled a few months earlier and Charlotte felt it was time for her to leave because the show she started on was cancelled. If she had stayed on DS, she would have left anyway in 1986. Interesting that on DS Mrs. Garrett was there only for a season and a half while on the spinoff show she stayed 7 seasons, and that when she guest starred on the episode Out of Peekskill she was the only DS character still "on air" after its cancellation.

I think it's just coincidence, because she's never mentioned DS when she talks about why she left FOL.