Adams has a graph showing various films' positions on IMDb's Top 250, which are determined by users rating each film on a scale of 1-10. There are day-to-day fluctuations, especially when a film is still current, but Milk was holding steady at around #180. Then, as soon as the Oscar nominations were announced last Thursday, Milk took a serious nosedive. Within 24 hours, it had dropped from #182 to #211. The decline has continued, and it now sits at #223.
Why the sudden and huge backlash? Surely all those people giving it a "1" rating didn't all hurry out and see it -- and hate it -- the day the Oscar nods were announced. Either they'd already seen it and hated it and weren't motivated to express their disdain until after it got so much Oscar attention; or else they still haven't seen it and hate it anyway. When so many people cast "1" votes in such a short span of time, it's hard to believe those are the true opinions of actual viewers of the film.
Adams' theory is that these are gay-haters who can't stand the idea of a gay-themed film doing well with critics and the Academy. He notes that a similar drop-off happened with Brokeback Mountain three years ago. There's no way to know if that's really what's happening, but it's a plausible theory, especially with things like California's Proposition 8 putting gay rights in the news so much recently.
But as some of Adams' commenters have noted, it could also be a backlash from people who hate Sean Penn (quite plausible) or Gus Van Sant, or an organized campaign by a group that has some other reason for disliking the film. Then again, the most obvious answer is usually the right one, and regular IMDb movie-raters aren't exactly renowned for being mature, circumspect citizens. You need only look at the negative ratings for movies that haven't even opened yet to see that.
And even if you don't like Milk, giving it a "1"? Meaning it's one of the worst movies you've ever seen seen? Come on. If that's what you think, then you need to see more movies. I recommend you start with the Martin Lawrence shelf.