There's been a lot of speculation Kristin Cavallari and Jay Cutler are getting divorced because of an affair with another woman, but we've learned that is NOT the case and the dispute between the 2 involves custody over their 3 children.
Sources with direct knowledge tell TMZ ... Kristin was "surprised" that Jay filed first -- she didn't know he was doing it. There's one thing in his divorce petition that set her off ... his claim that he was the "at-home parent" who was the "primary caretaker" of their kids.