"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." - excerpt from Memory Alpha: Gates McFadden. - Archduk3:talk 10:56, 11 August 2009 (UTC)
Off the official record, I heard that she alleged sexual harassment against someone on the staff and left when nothing was done, but he was gone by season 3 so when Muldaur wanted off they brought her back. "Bad blood" may be an attempt to say that indirectly.
The correct answer is: Because she was pregnant.
The late Herbert J. Wright cited Hurley as one of the reasons he left TNG. He described Hurley as "basically playing drinking buddies with Gene." (Captains' Logs: The Unauthorized Complete Trek Voyages, p. 224) Tracy Tormé also named Hurley as one of the causes of his departure, after numerous disagreements over Tormé's scripts. (Captains' Logs: The Unauthorized Complete Trek Voyages, pp. 170, 177-178, 181-182)
According to Rick Berman, Hurley was the reason behind Gates McFadden's departure from The Next Generation in its second season, as he disliked her acting and "had a bone to pick with her." After he left the show in the third season, McFadden was invited back by Berman. 
I remember her being on the show while she was pregnant. They just covered it up with her smock ProfessorTrek 16:47, August 23, 2011 (UTC)
She was pregnant during the show's fourth season, so that had no bearing on her absence during the second season. Quornhog 12:20, October 13, 2011 (UTC)
Yes, Her son was born in 1991.
My mom told me about that. Apparently she was in "The Hunt for Red October" for about 5 seconds and that's why we got Pulaski.
== This question pops up at least twice a month here, I suggest that you check her page at Memory Alpha for clarification. Basically she was not satisfied with the way her character was being developed during the first season and there was some bad blood between her and the producers. She wasn't the only cast member to complain about this; Denise Crosby, Marina Sirtis, Micheal Dorn, and LeVar Burton had apparently made similar complaints, Denise Crosby of course left before the first season even ended.-Cpthunt (talk) 07:51, August 14, 2012 (UTC)
This is a question that Gates was asked at every Star Trek conference she ever attended. Generally speaking, she didn't provide many details in those days (to protect her job).
The simple truth of it is that there were numerous issues in the production office and nobody thought the series was going to last. The production office was in utter chaos during the first two seasons. After some staffing changes just before the 3rd season (Maurice Hurley getting fired for his antics) STTNG took off and became a mega-hit.
One of the producers of the show at that time, Maurice Hurley, had a personal issue with Gates and just wanted her gone. Hurley and McFadden did not getting along even a little and Gates didn't like the way her character was being developed. Hurley, who often behaved like a tyrant, was tired of complaints (Gates was not the only actor who complained) and basically used his influence to have Gates McFadden fired.
Diana Muldaur, who replaced Gates McFadden, didn't like the fact the series used special effects to tell the story. In an interview on the subject, Muldaur said she like to be around actors and that this is the way TOS was. She didn't consider what she did acting. She basically read her lines and then the special effects told the story. By the end of the 2nd season, she wanted out. By this time, Hurley was gone (the studio got rid of him because of his behavior) and Gates McFadden was offered the chance to return. She took it.
With Hurley gone, her character, and those of some of the other actors, were given a chance to develop and the rest, as they say, is history.
02/10/2016 - 09:04am MST