In addition to the obvious misogyny, I'm also detecting some resentment of Twilight Sparkle's rise in status. Because Twilight's a nerd and all these Bronies are nerds too, and they think of nerds as being like the ~oppressed underclass~ (particularly in a school setting), but now that Twilight's moving up in the world and gaining power and authority, the Bronies are pissed because a) they're jealous and wish they could also escape the nerdy underclass and b) they see it as Twilight joining the ranks of the mean girls/jocks/cool people who are mean to nerds, making her a traitor. It's the old "you're relatable because you suck, just like us, so how dare you stop sucking!" complaint.

I'm pretty amused that anyone's surprised about Twilight's promotion. I mean, she was always Celestia's protege, from the very beginning, and she's always had way beefier magical powers than her peers. Plus it's in keeping with standard girls' show tropes for the lead character to become some variety of princess, but I guess the Bronies wouldn't know about such things.

Frankly, I've had a distaste for the Bronies ever since they coined that stupid nickname for themselves, because they felt the need to point out that they were different from those other MLP fans i.e. little girls.