what the coaches say. IMO, part of it is he's a really competitive person and he knows who the main rival is. Part of it is he knows what his fan base wants to hear and he plays to them a bit. And the last part is more speculative. His dad coached at BYU and then went to the NFL. When his stint in the NFL was done he wanted to come back to BYU. I have heard there were some hard feelings that LaVell didn't let someone go in order to make room for him. Utah hired his dad at that point and the family loyalty switched to the dark side a bit.
If you ask me, it appears he has more animosity toward BYU than he actually does.