This is what happens when you are in your final season,
they bring back characters who have left...
NYT: Downton Abbey Season 6, Episode 4: Guess Who's Coming To
And since Rose Leslie is freed up from Game of Thrones, it's a welcomed gig.
EBL: Women of Downton Abbey and Rose Leslie Rule 5
Instapundit: The Dowager Countess goes rogue, Victor Davis Hanson: The return of traditional values
Rule 5 and FMJRA