I think a lot of the youth that have only lived in Utah use it as a way to rebel against parents, friends, local church leaders etc that probably push BYU. And then they grow up liking the Utes. Where as if you go through the high school years in the mission field you tend to want to associate with the comfort BYU and a lot of fellow LDS kids bring.
A lot of kids try to go against the grain just to be different and seemingly make their own identity. No harm in that. But some do take it to the extreme.