I will attempt to answer the question that was asked in the thread title without taking the video into consideration.
There are multiple reasons anti-bronies exist, few more obvious than others. I think that one of the most prevalent and blatant reasons is lack of knowledge about the the fandom, or to put it more succinctly, ignorance. Many anti bronies see fans of the show as 'fat nerds that watch colorful ponies' or something like that, what a lot of them don't seem to understand is that even if all bronies are 'do nothing nerds' flaming about them on the internet would do nothing to help their cause nor would it suddenly compel the fans of the show to stop liking it. Another reason one may decide to become an anti-brony is through the gross generalization of an entire group because of a few individuals. It is inevitable for a large community such as this one to have a few individuals that feel the need to attack anybody that does not share their point of view. Some people are attacked by these individuals and feel the need to reciprocate the displeasure back onto the entire community, I think that the theory that bronies only claim to like to show because they are trolls stems from this. In conclusion, I think that not all anti-bronies are willing to go out of their way to insult bronies; same applies when the two groups are flipped, both ends of the spectrum have their bad sides. There is no point in attacking anti-bronies and there is no use for anti-bronies to attack bronies, actions such as that are contrary to the morals FiM teaches and the ethic of reciprocity.