From an interview with StarTrek.com in 2012:
If you had the chance, today, knowing what you know now, to make that
decision again, would you make the same choice?
Crosby: Yes. For me, I was miserable. I couldn’t wait to get off that
show. I was dying. This was not an overnight decision. I was grateful
to have made that many episodes, but I didn’t want to spend the next
six years going “Aye, aye, captain,” and standing there, in the same
uniform, in the same position on the bridge. It just scared the hell
out of me that this was what I was going to be doing for the next
X-amount of years. I think you have to take your chances. I was really
young. I didn’t have to make house payments or put kids through
private school or support people. I was free to make those kinds of
decisions. I’d been in acting school really dreaming of playing all
kinds of different things. Whether it’ll happen or not, you don’t
know, but you’ve got to give yourself a chance. God forbid you go
through your life thinking, “What if?”
From the second part of that interview:
You asked to be let out of your TNG contract, and you were. How
surprised were you, then, when you were invited back for “Yesterday’s
Enterprise”?
Crosby: I was surprised on so many levels. First of all, my character
was dead. But, I did leave on really good terms. Gene Roddenberry and
I met one on one in his office. There was no animosity. I don’t know
that anybody really wanted me to go. I think it stirred up a lot of
things in all the other cast members. I’m not exactly sure what, but
you’ve got to question your own commitment or your own place, what
you’re doing there. I think it stirs up stuff. However, Gene and I
were very clear about what was going on. He said to me, “I don’t want
you to go, but I get it. I get why you’re leaving. I was a young
writer at one time and I was hungry and I was frustrated, and I get
that.” We hugged and that was it. He got where I was coming from.
The rumor that her Playboy spread was responsible for her being fired is false:
First, while Crosby appeared in the May 1988 issue of Playboy, the
spread was a reprinting of a pictorial that she had done for the March
1979 issue, early in her career as a model, with the selling point
being “Here’s Bing Crosby’s granddaughter … nude!” So she had already
posed nude when she was hired for the series. (Playboy reprinted the
photos nine years later without letting Crosby know. She told People
magazine at the time, “It’s a bit exploitative of Playboy to do that,
I suppose. But I’m not bitter about it.”)