Most of the hate comes from cringe, misconceptions(the common one being that ALL furries are zoophiles and/or want to be animals, when it's really just a minority), someone having a bad experience with someone who just so happens to be a furry and pushing it onto the rest of the fandom, or just absolutely no reason at all.
Of course, I cannot deny that the fandom does have a dark side, as nearly every other fandom does. Nothing in the world can be perfect.