OK, but what if they did? Now we're left with the question of why, and figuring that out is surely going to be interesting.

"I ."

"You're about to, when you change your mind. What made you change your mind?"

It's a powerful tool. It can be overused, but it's good for bringing people into the right frame of mind.

Maybe something happens that's more urgent than the trust issue. Maybe they see a tattoo on another character that has meaning for you. Maybe they just realize it could be useful to be in the party for now. Whatever it is, they are solidifying the team while also taking more authorship of the story.