@Meglos
I'm pretty sure this was made pretty clear if you've watched the season closely?
Brad is tight with both Tai and Sierra. Tai's initial desire was to have the pecking order be Hali, then Debbie, then Caleb. In her original season, Sierra wasn't a huge immunity threat, but she placed solidly in many individual challenges - she's a tall, athletic woman - definitely worth keeping for tribe strength over someone like Hali or Debbie.
Meanwhile, Brad was concerned about the Kaoh Rong three potentially teaming up later on down the track, and in a game of numbers, he wouldn't want that - no matter how close he is to Tai, who himself mentioned how he got caught in positions where he could hurt friends either way in the last game he played.
Honestly, the pecking order on that tribe seemed to be Brad - Sierra - Tai - Debbie - Hali/Caleb; but Brad made clear why Debbie wasn't ever considered - she's loyal.
So it ultimately came down to Hali or Caleb. Brad masked his concern of a Kaoh Rong 3 from Tai by labeling Caleb as a potential threat. There was also Caleb's issue that he didn't have many connections left in the game, outside Malcolm. This would've prevented Tai from really running with him, because there's no need to stay loyal to someone on the wrong side of the numbers in the grand scheme of the game; so I guess Tai cut his losses there. Meanwhile, Hali and Sierra distancing themselves from each other worked as they weren't seen as a power couple that needed to be split up. And then at Tribal Caleb shot himself in the foot by playing up all his strengths, only to have Hali turn them against him by showing why that would make him dangerous down the track.