Well, humans don't like the thought of someone doing something not ordinary, or cool. Humans always have to be with everyone else. And if there are any outcasts they are shunned by society.
This is a true phenomena, but not the one that applies.
The reason why people dislike bronies on the forums is that it has grown to be one of the longest and strongest fandoms to ever emerge on these forums. What happens is that a person looks at it who lacks any form of unique trait or ability and says, "Because this is popular, if I dislike it, I'll be unique!". It's the same reason why people become hipsters. This does not ALWAYS apply to everyone who dislikes it, some people strictly hate the fact that it's become loveualized, and some people just think the show doesn't deserve the credit it's given.
Either way, none of those classifications holds enough bearing that a person should dedicate hours of their time trying to get rid of it. Anyone who has already invested such time should be laughed at.