In this case they came up with "Lien left of her own free will" and "mutual agreement", later on changed to "we couldn't come up with ideas for the character" and "mutual agreement" which indicates that they actually kicked out the actress/character.
Just playing Devil's Advocate here: Couldn't that also mean that the reason she wanted to leave was that her character wasn't going to get many episodes?
Imagine she went to the producers, and said "I'd like to stay, but if I'm not going to get as much airtime as Tuvok I'm not sure it's worth the obligation. I keep turning down movies because 26 weeks a year I have to be available to film Voyager, but I'm not even used as an extra in every episode. Is my character going somewhere, or should I start looking for other work?"
And the producers reply, "You know, we have no idea how to write for that character. I think, at best, we might get you two episodes per season where the focus is on you. I totally understand why you might not want that, and if you'd like to leave the show we can write you out in a way that would let you come back, and write you a glowing letter of recommendation."
A slightly better fit is that she just wanted to leave, and at first the producers were upset about that (so they said "it was a mutual decision"), but with a little time they realized that it was really their fault, but it wasn't anything they could change ("You know, we never could write for that character. She wasn't getting much screen-time, and it was only going to get worse. She thought Seven of Nine meant she was going to get pushed out of the picture completely, and honestly she was probably right about that. Leaving was the right choice."
In my experience, "mutual agreement" usually means it was specifically someone's fault, but nobody wants to say whose. It may have been that the producers were forcing her out and she doesn't want to say so. It may be that
she wanted to leave and they begged her to stay.
Unrelated note: I see at the imdb that most of her work before Voyager (and all of it since) has been voice work. Maybe she decided that live-action acting didn't agree with her somehow.