I've finally played through the Summerset expansion, and know I finally know why Nocturnal acted a bit out of character when comparing her behavior from previous games. I imagine any of the Daedric Princes would act out of character to get infinite power. However, the expansions Vvardenfell, Clockwork City, and Summerset are antagonized by an ALLIANCE of three Princes. Clavicus Vile got Vvardenfell, Nocturnal got Clockwork City, so I assumed that Mephala would get Summerset. It would also make more sense for her to betray the Triad since her sphere of influence includes lies and betrayal. It also causes my OCD to itch that Mephala wasn't the last boss in this expansion trilogy.