The Saga of Gates McMuffin

Came across this explainer of why Dr. Crusher left the show, then later
returned:

Gates McFadden revealed during a convention appearance that a producer (she
refused to identify him, but it was eventually revealed to be Maurice Hurley)
gave Gene Roddenberry an ultimatum: 'She goes or I go.' It was a choice
between losing an actor or going up against the studio like he did in the
'60s, and Roddenberry just wasn't in the condition to do that any more.

Incidentally, it wasn't entirely McFadden that caught Hurley's ire. Hurley was
brought on by Roddenberry's lawyer, Leonard Maizlish (who was himself
thoroughly despised by almost everyone involved with the series); Maizlish had
no authority at all to hire or fire anyone, but the studio did very little to
rein him in, even when Maizlish deliberately violated Writer's Guild rules by
re-writing scripts.

As for Hurley, he'd had no previous science fiction experience, was openly
disdainful of Gene's optimistic view of the future, and clashed regularly with
almost everyone. It was a series of contentious arguments with Gene
Roddenberry himself that led Hurley to leave the show at the end of the second
season.

Diana Muldaur's attempt to recreate Spock and McCoy's famous verbal jousting
by arguing with Data just made Pulaski look prejudiced and mean-spirited, so
once Hurley was gone, Rick Berman personally invited McFadden back to the
show.