On the surface a very strange move ...
On the other hand, what is BYU at this point?
They made the decision to go independent but have essentially ZERO chance of getting to a high level bowl as they stand. I'd imagine that Bronco looked at the program and figured he got it about as far as he could get it.
When you look at UVA, it's a very good school with a reasonable recruiting base to pull from. He's a successful coach and his track record is definitely better than that of Fuente at VTech. No reason to think that he can't be remotely successful at UVA.
If anything, what this move tells you is that people that have aspirations need challenges. Bronco was probably safe at BYU as long as he kept doing what he was doing. Not too much to argue with. But he knew he maximized his potential. So instead of being comfortable he looked to push his bounds. Honestly, that's what you want to see out of a coach. It's what you saw with Petersen when he realized he maximized what he could do at Boise and went to Uw.