I once read something that hypothesized the hate came from a joke from Colin Quinn's Comedy Central panel show: "I think music can make people violent. Like, Nickelback makes me want to kill Nickelback."

It ran on promos when CC was at its peak popularity, and it spread like a proto-meme when online culture was just getting started. I used to jam to butt rock like them all the time back then and thought the hate was both hilarious and justified.