Like everyone else, she saw that the storytelling improved and she wanted to be a part of it. I don't see anything shameful about that.
Nor I. In the first season she had a great speech in the premiere, a goofy episode involving closeups of her posterior and trying to have sex with Data, and another goofy episode where she fought a misogynist culture. All of that happened in the first four episodes, and her involvement in the rest of the season was mostly security officer stuff with an occasional memorable scene peppered in (I fondly remember her "penalty box" speech), but nothing that really focused on her. Heck, even "Skin of Evil" wasn't really about her either. Harry Kim had more to do than she did in his first year.
I'm sure she understood that this was an ensemble show, but even that level of involvement would make her justifiably worry about what she'd be doing in the second season onwards. Her character had some pretty tragic origins, something that the writers may even have wanted to steer clear on (really, who wants to write about "rape gangs"?).
Really though, her departure strengthened the show - one less character to juggle. I would have liked to see more Tasha, since what we got was good and well delivered. But Crosby gambled on moving on, as actors often do. IMO she lost (yay, "The Eliminators"!) but hey - that's the biz.