I'm racking my brain trying to think of the reason but I'm sure there was one. If you have a read through the feeds thread I'm sure I or someone else would've been commenting on their plans.
I think DTM is along the right lines though. Nikki constantly fluctuated with her like/dislike of Cass, she was supposed to be in an alliance with her, Tim and Joel but would have conversations with the brothers and Kelsey about how she couldn't stand her and wanted her out
then they'd make up and she'd drag Kelsey to Cass, it was a MESS. I think that close to the endgame she just became expendable and too much of a risk, with like six people in the house if you can't rely on someone's vote cause you might've annoyed them that day it's not ideal
Nikki's eviction was just her being a victim of circumstances anyway. Had things not played out that way, I think there's every chance she'd have been dragged to the F2 by any one of them