I think you may have answered your own question. According to the googles, Kelly makes 1.665 million a year? Can that be right? Fuck that seems low. He's been there for 11 years and just signed a 4 year extension, so I'm guessing if he could have milked more money out of ND, he would have by now. At first blush, I don't know why he'd leave ND to go to Texas, but if the difference in money is somewhere around 4-5x, that could be a pretty easy call for him.
Peel back the layers a bit, and Kelly's probably a great fit at UT. Here's a good article about academic recruiting restrictions at ND. I can easily see the UT admin loving that philosophy and sales pitch, with where UT wants to be academically. Only difference being that they don't really have the same restrictions so BK could sprinkle in some higher level recruits who would have never gotten into ND, while still pitching the same opportunities.
The more I think about this, the more I think Kelly might be a great plan B to Urban. Maintained 2+ programs, has experience handling high level recruits and player egos, and high level programs, boosters, pressures, etc. Has made some great assistant hires over the years. Isn't an immature jackass who just happens to be the hot flavor of the year. Okay, he hasn't won a National Championship, but nobody we can get who's worth a damn has. So, outside of Urban, I think he'd have to be in the conversation for the obvious first choice. I think I like it.