Wyoming fans hate being treated like ignorant, sheep-abusing hicks. Utah doesn't like to be called a commuter school. Aggies hate the joke pretending nobody knows where they are. And BYU gets irate about jokes about polygamy and the like, and they have an unusual adversity to salty language.
It gets under your skin. That's why they do it.
Also, you literally just implied that Wyoming fans have sex with sheep. You can't see how this is just as offensive as F Mormons?