Gates McFadden was fired after first season because of a personality conflict between her and the head writer. Sexism apparently played a role in this mutual dislike.
Diane Muldaur (Pulaski) was brought in to be more of a Bones-esque character. Muldaur reportedly did not like the show too much and never intended to stay on for more than a season. Her character was declared a failed experiment and she was not offered a renewal. Which was cool with her as she went on to LA Law.
The writer McFadden had an issue with was fired after season 2 and McFadden was offered the role as Crusher once again.