It's mostly the extreme feminist types, who have reasons for hating all men, they just happen to find somewhat less ridiculous reasons for hating brownies (sorry, but you have to admit that a relationship with cartoon animals is not exactly something that reality can, uh, DO. again, sorry. nothing against bronies), and less ridiculous reasons means larger base of people who hate bronies.

From my experience, they tend to butt in in places where MLP has zero relevance. Example: There was a SFM animation on youtube. The audio came from an abridged series video from pre-2010. It mentioned the word “ponies”. Suddenly, a flood of bronies started spamming comments like “I’M A BRONY AND I THINK THIS IS THE GREATEST THING SINCE BALD EAGLE FARTS!!!”
If fandoms were dicks, then theirs is springloaded with a hair-trigger at the slightest mention of anything equine or cartoon related, and they wave that shit around with zero self-respect or dignity.