More likely Leia is introduced by the DN as a strictly background character to begin with - the whole "rescue the kidnapped princess" thing being so much of a trope for trying to get control of a campaign back it's laughable. Consider after all that her initial interactions are pretty much all with Vader, who will presumably still be a PC, and she doesn't really become a playable entity until the breakout at best.
Then I can see one of the other players deciding to take her over. Either Ben when he loses Obi-Wan. Or even Annie when she decides she doesn't like where Vader is taking her. It's much the same after all as when the DM introduced Shmi as a character for Annie, with Anakin as fluff, only to be totally subverted.
It's somewhat moot as to why tje DM would let a player switch characters that way (Ben's onscreen death is pretty much by his choice in the movie, and I don't see the sdreen caps omitting that) since there is ample precedent for it not only with Annie but with Sally ping-ponging between roles.