I was also wondering if Tai knew exactly what he was doing, but Jeff (in EW) and Nick (in his post-game interviews) say it was a mistake.

Nick says that at the last minute, Julie/Aubrey wanted to switch the vote to Jason vs. Nick. This would make more sense from their standpoint since Nick is their ally and it would split up the Jason/Scot pair.

According to Nick, Cydney and Aubry would have gone along with this, but Debbie says it's too late to switch the target and they need to stick with Nick because Joe was already told and is set on Nick and he'd get mixed up and possibly vote incorrectly. (This cracks me up, especially after the snoring scene last episode.) So the girls stuck with Nick but Tai didn't hear that part and apparently still thought the girls were voting Jason and went along.

It will be interesting to see how Tai deals with the aftermath of his mistake. Guess Scott and Jason really don't have much choice but to stick with him since they need his idol for any glimmer of hope in making the super idol.