I agree with the skepticism about Gundy leaving oSu for Tennessee.
But reasons that he might leave could be: he's oSu's best-ever head coach, and yet OU's dominance (87-18-7) of the Bedlam series has been even greater since he became head coach (11-2) than it was previously; he believes he's taken oSu as high as it can go, while Tennessee would seem to have a big upside; he's tired of the alternating complaints and praise from oSu's sugar-daddy T. Boone Pickens; or he's just ready for a new challenge.
He has done some odd things this year that make it seem as if he might be going through a mid-life crisis. He's got his mullet hairdo. He's worn a rasslin' singlet to do a promo for oSu's most successful-ever program, he's done an "I'll do it if you'll do it" act with QB Mason Rudolph that resulted in them both taking their shirts off at a pep rally.
But I think that he genuinely loves his alma mater, and that he will most likely stay in Stillwater.