It was just a knee-jerk reaction to how omnipresent they were. That was the very end of when people actually listened to the radio in the car or whatever so you'd still get exposed to whatever was on the least worst station in your area.

To be honest, I just got thoroughly fucking sick of hearing rockstar or whatever on the radio. It's not even offensively bad or anything, it just got drilled into that lizard lobe of my brain.

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.