Originally Posted by horatio
Wasn't Roddenberry still in charge during the third season? Anyway, I guess he brought Muldaur in, knowing her from TOS.
This is what MA (http://memory-alpha.org/en/wiki/Gates_McFadden
) has to offer about the subject, but they give no source for their information:
McFadden left the series during the second season, replaced by Diana Muldaur as Doctor Katherine Pulaski. Officially, the producers felt that the character of Crusher had not developed as well as they had hoped. Unofficially, there were some disagreements between the show's producers and McFadden as to how the character should develop, which may have led to some bad blood between her and several executive producers, resulting in McFadden's being released from her contract before the second season. But thanks to a letter-writing campaign, support from Patrick Stewart, and a personal invitation from Rick Berman, McFadden was brought back to the TNG cast for the third and subsequent seasons.
The Continuing Missions TNG retrospective says broadly the same. That she had been 'persistent in asking for expanded parts and rewritten dialogue' and for strengthening her character. So, at the end of the season she was released from her contract.
However, Pulaski didn't work out either, and so McFadden was invited back to reprise Crusher. Reportedly (and perhaps due to a less than cordial exit first time around) the rumour was she was hired back at a lower salary than the rest of the cast.
Originally Posted by Jed Starkiller
I thought it was because she was pregnant
She didn't have a pregnancy until season 4 was filming.