It's not hating something because it's popular, it's hating it because you hate it. If it wasn't popular, then you probably wouldn't know about it, and therefore not hate it. Because it is popular, you are more likley to experience it, and if you hate it, you hate it.
It's the whole 'hating something because it's popular' thing really isn't it?
Although the reverse of that is you now can't hate someone who is popular because you get accused of trying to be cool. It's a vicious circle of hidden ulterior motives.
Dunno. We're tribal creatures, and we work on shared values. If someone you dislike (e.g. Unwashed Masses) likes something, that makes it less likely that you'd hold similar values. If you think that you're a genuine free spirit and not influenced by the opinions of others you admire or aspirate to be like, you're either deluded or strange.