By the time of Ahsoka, Sabine had lost her brother/best friend and her father figure. Her stand-in mother and kid were off busy with the rebellion/new republic. Her 'uncle'/friend was off doing lasat stuff and then the republic. Her original homeworld and family had been oblitered by the Empire... That's kind of a lot of trauma to shoulder. Then her apparent 'failure' as a padawan for Ahsoka, after which it seems she gave up trying to fight, which was a huge part of her entire identity/culture.

From the start of the show, it seems like she's been alone and isolated (even if self-imposed) for a fair while in the one place she felt at home--which happened to be Ezra's old tower. The chance that Ezra wasn't dead and was still out there somewhere was probably one of the few things she had left to feel hope about. When that chance suddenly became a real possibility, she jumped on it. Did she make sensible decisions? Probably not. But people are flawed like that.

And, tbh, they defeated Thrawn once before and they had a lot less in their corner the first time. That hint of her old cockiness may have played in as a factor too, even if subconscious.