Its too much to expect logic from Priyanka. Everyone and their dog knows Priyanka and Ankit are going nowhere, same can't be said about Gautam and Soundarya.
By bringing Gautam and Soundarya in nominations, she has made sure either one of them are getting evicted this week. Its likely Soundarya, and once she's gone, Gautam would blame Priyanka for it and might also go back to Nimrit and shift to Sajid's group. In short with this move, Priyanka has broken this group, and there's no guarantee Shalin and Tina would join her after Soundarya and Gautam are gone.
If Priyanka had nominated herself and Ankit along with her, she would've gained trust of Gautam and Soundarya and also could throw Shalin out of the house.
And Priyanka has done this just after last WKV where she was ranting about how she has such a giving nature and she always thinks about others before herself.Yeah we saw it in this nomination task. Lets even say there's nothing wrong with playing a game and first thinking about yourself in Bigg Boss. But then drop this holier than thou attitude of "main kitni achhi hoon, main kitni mahaan hoon blah blah."