There's a concept in psychology called "Dunbar's Number" (which is often called the "
Monkeysphere"), which basically states that there is a cognitive limit to how many personal relationships that we can sustain (which is usually around the 150-200 range). Everyone outside of that becomes a person-shaped "other". Which is, incidentally, how people can be totally racist and yet have black friends. So while they think every other black person is a looting welfare queen who's going to rob, rape, and kill them, they think their black friend (let's call him "Bill Misth") is an exception to the rule. "Bill's not like them though, he's a good guy."
So, how does this apply to band fanbases? Well, think of it this way: In a smaller group, it's easier to "personify" the other members of that group. That's not a faceless person, that's Bill Misth. As the group starts to get bigger, it becomes more difficult to "personify" all the members of that group. It's also more difficult for other people to personify
you. If you're in a chat room with 8,000 people, what chance is there of anything you say actually being heard? You're just another blip in an endless stream of noise. So, the band's new found popularity actually becomes a
threat to that sense of community and to that person's role as an individual in the community. As the fandom gets bigger, everything you and Bill Misth say loses importance, and eventually becomes just another piece of noise. Now, obviously, that's frustrating. So, in your frustration, you try to find a source for the loss of community, and voila! It's the band's new popular song and the new fans who aren't really trve fans!
As for hating on artists like Taylor Swift or Skrillex, the hate typically stems from one of two things: that "Monkeysphere" effect I mentioned earlier or "overexposure". Hating things that are typically super popular makes the fanbase of the smaller group feel more "special" and more like individuals. The fans of that popular thing are just a faceless, stupid mob who only like that music because everyone else likes it. They don't appreciate
trve art like [insert niche band here]. Going against the grain helps them and their community fell more "special" and more "their own".
Here's a really good article about how this happens and how it breeds radicalism. As for overexposure, if you're forced to listen to the same song over and over again on the radio or at the supermarket, eventually you'll come to resent it. It's like how friends can get along really well and be super close normally, but they end up just bickering with each other when you move in together and have to be around each other constantly. Phil Collins/Genesis is a bit of both, the earlier fans felt their special community degrading with the huge influx of new members, and they were so present on the radio that eventually people eventually just got annoyed.
Also, it goes without saying that this doesn't really reflect on the quality of the music itself. Just because something's popular doesn't mean it's bad, and just because something's niche doesn't mean it's good.