I think Punk may have had a Bret Hart like epiphany along the way at a certain point. Losing seven figures doesn’t help anything, but his answers from Starrcast would point to him thinking about this for a decent amount of time. He was super political and respectful in that he said he’d talk to anyone and that he’s changed and is at peace with everything and the past is the past. He essentially rolled out the red carpet for WWE to come talk to him again. Maybe they didn’t and he decided the Fox gig is good to open the door again.
The epiphany being that WWE is such a giant force that they control how your legacy is perceived and viewed. Maybe CM Punk realized that he doesn’t want WWE smearing his legacy at a later date, or that he wants to be remembered a certain way in a more positive light, or that he truly wants to end his wrestling career with a proper final run. Again, the money lost in the lawsuit would be a huge blow to anyone and that would be another deciding factor, but I think he’s seen the changes and wants in because it’d make him happy, he has the itch again, and it’s the only place to truly go and properly end his career and cap it off with a Hall of Fame induction and rake in royalties for the rest of his life.