There are two possibilities: He lost his tenure because there was an anonymous, incorrect peer review negative towards him. His work was actually good. In that case he should sue the university to make decisions based on anonymous, incorrect peer reviews.
There's a third, more mundane possibility: he lost his tenure because he quit. When he lost his new job offer, he went back to Wayne State asking for his old job back, and they said no. The devil is in the details here. If he had a written employment and tenure agreement with Mississippi all signed and finalized, he would have a damn good case against Mississippi. TFA is not clear on this point, but I would hazard a guess that he got an informal notice that Mississippi intended to hire him, quit at Wayne State before the offer was official, and then Miss yanked the offer before they were legally committed. This kind of shit happens all the time. So sorry.
Moral: never, ever, quit your current job until the ink is dry on the legal papers for your new one.