He could be just hoping that Spencer doesn't win immunity at this point, but I can' help but feel it was a mistake. Because if Spence wins just one more time, Tony's done. At final four, Kass and Woo have no reason not to vote for him and test his idol claim., and with Spencer guaranteed at final 3? Who wants Tony to be the third there?
Trish would be his only protection at a final four with a safe Spencer, and serve as a buffer to make certain that they're wouldn't be any slick last second decisions even if Kass or Woo pulled out an immunity win.
Voting out Trish shows either great confidence in his allies, a gamble on Spencer's chances of winning, or just out and out fear of Trish.
And I don't see the reason for the fear.
Sarah, Jefra, and maybe LJ (maybe, but doubt it) are the odds on favorite votes for Trish over Tony. Trish isn't Natalie White, and Tony isn't a Hantz. Morgan will vote in a way that makes her look good on television, so solid money would be for her making fun of Trish and Kass because she's a vile character and not an actual person.
Jeremiah is a coin flip.
Tasha votes for who she thinks played the best game, and in a final 3 with Tony, Trish, and Kass/Woo? She's voting Tony.
Woo I think would be bummed, but good hearted enough to vote for Tony.
Spencer votes Tony all day long.
What has Trish done on the island to appear as any kind of threat to the players? We've seen her big moves of taking out Cliff, bringing in Kass who was planning on flipping anyway, and standing by while Tony took out his own alliance and telling everyone to maintain loyalty. But has anyone playing the game seen her do anything other than yelling at people and being repeatedly blindsided by her partner?
Trish is a threat if people like her or really hate Tony. Otherwise she's one of many older white women that thought they'd beat their Russell by doing nothing of note and standing next to him.
She's nowhere near the level of guaranteed win that Spencer is if he makes final three, and I can't figure out why Tony would see losing the protection she offers at final 4 an acceptable loss, to mitigate a slight spite vote threat.