Hold a minute most of the time when WWE stars want to leave and go to TNA it's because they can still get a good payday but work less. I remember hearing Punk was getting tired of the schedule WWE had him working and since he is not considered a top superstar by most WWE higher ups I could see him splitting to work more on his terms. Of course that doesn't mean he'd work for TNA, but I've seen stranger stuff happen in the wrestling world. Punk's attitude makes it look like he wouldn't want to work in such a place, but his character would thrive in the main event scene in TNA. Now that he has made a big name for himself he'd be a top player in a place like TNA.
Punk always has an agenda. He knows what he wants and he'll do what he wants. That's it in a nutshell.