Bronco and Holly Mendenhall wanted out of BYU.

To your point, The fan base was split and still is.

Bronco has explained it this way: He and Holly see themselves as boulders. They rebuilt the BYU program after Crowton and the last lean years of La Vell.

After they rebuilt BYU (and the evidence suggests that due to independence, neither Bronco or anyone else has been able to build it any higher) They wanted a new challenge of building something significant. They have found that at Virginia.

This is from Bronco’s own lips. There is no need to keep offering speculation as to why they left when they themselves have explained it very simply and clearly.

I see no reason to doubt that Bronco is being untruthful.