I remember the first time BYU and BSU played. I was a BYU graduate and a loyal fan of both teams (I always say that I root for whoever is higher ranked, unless they meet in the playoffs, and then I root for BYU) and because of that I knew both teams.
Anyone following both teams knew BSU was going to kill them that year and I told an sister in our ward who was a huge BYU fan that. She scoffed but she wouldn't look at me or talk to me after the game. Ever.
I think a lot of BYU fans had their pride hurt and thus hate BSU. That's sad and a really bad look, but kinda funny, too. We humans are kinda weird.