Believe it or not, it wasn’t Ashton’s cheating that finally made up Demi’s mind!

Demi Moore‘s decision to divorce Ashton Kutcher may not have been surprising — it’s been anticipated ever since reports of his affair with 22-year-old Sara Leal emerged in September — but the reason behind her decision comes as a major shock. Sources on the set of Two and a Half Men tell HollywoodLife.com exclusively that it was Ashton’s careless tweet about the Penn State football scandal that actually pushed her over the edge!

“It looked as if things were progressing, and then of all things when Ashton messed up on Twitter,” our source says. “It was another thing to get Demi upset. She believed he really didn’t care about the details of things and the embarrassment of that reminded her of the embarrassment he caused her with the cheating. It basically opened the can of worms again resulting in Demi finally deciding to take on the divorce.”

Their relationship had already been rocky while Demi was off filming LOL, our source adds, but friends still believed the couple would work things out. In fact, Ashton’s cheating allegedly helped their marriage — briefly. It supposedly made them “realize that there was still love in the relationship and they wanted to work things out. That is why you started seeing them camping, her on set and other moments of them together trying to repair their relationship woes.”

Then came the Penn State tweet, in which Ashton said he found it “in poor taste” for Penn State to fire Joe Paterno – despite having no knowledge of the sexual abuse allegations.