Redditor babycarrotman has done some excellent data mining in an effort to understand when the hate for Nickelback began. The answer seems to be May 5, 2003. I quote:
After reading [this post] I suddenly realized, very few people there know the primary moment that popularized hating Nickelback. And looking online, very few other people seem to know the answer, either.
People have argued that it's because their lyrics are derivative, or their music is all the same or some more sophisticated argument about popular perception of their music (see the Cracked article and The Village Voice).
I submit that hating Nickelback, however, has a much more prosaic origin: an overplayed Comedy Central promo.