I'm joining in late here but....Garrett Wang was chosen to be fired because of his behavior, constantly being late and such. He was even suspended for a few episodes. But popularity saved him
As for Lien leaving the show, she had a 3 year contract, after season 3 it was up and not renewed. She wasn't upset about it and she moved on with her life
Jeri Taylor admitted, "These things are never cut-and-dried. It's never one thing. I think Jennifer had a wish to move on, and that coincided with some thinking here. There has not been any rancor or unpleasantness about it [....] I know she will go on to a wonderful career [...] and I will miss her very much." (Cinefantastique, Vol. 29, No. 6/7, p. 113) Taylor also said, "It was a mutual feeling [....] [Jennifer Lien] just felt her character wasn't going anywhere, so it was a very amicable and mutual decision." (Star Trek Monthly issue 31, pp. 4 & 13)
Those in charge changed their minds and kept Wang, not because of some common "popularity" but because of a silly poll among the staff of a glossy magazine where Wang ended up among "the 50 most handsome".
Jeri Taylor's rubbish about "mutual feeling" and that awful word "mutual agreement looks like a typical propaganda phrase. As a matter of fact, some years in the entertaining business has taught me that everytime the words "mutual agreement" shows up, then it's foul play involved.
What I know, Jennifer was very upset when she was told to leave and the rest of the cast weren't too happy about it either.
If there had been some truth in Mrs Taylor's statement, Then the whole thing would have been explained in the same way as when the actress who played Caitlin Todd in NCIS told the world why she had been written out of NCIS. In the "Lien case", it was no comment at all from Jennifer Lien herself for a very, very long time. Only some vauge comments from those in charge which first inclined that Jennifer Lien had left on her own free will, then later on changed to "the character wasn't working".
If "the character wasn't working", then the writers didn't do the job they were paid for and should have been fired. If there was another reason, then Mrs. Taylor isn't telling the truth. I find it amazing and almost impossible that the writers, including Mrs. Taylor herself all of a sudden lost their ability to write for the character Kes just at the beginning of season 4 when they had managed to come up with good stories for the character up to the last minutes of season 3 (and in Mrs. Taylor's case even after Kes was dumped which she did show in the book "Pathways").
As for the details, I guess that Guy Gardener
hit the nail on the head:
Wang was fired to make way for the hot chick.
Lien was fired to make way for the return of Wang.