These people don't necessarily hate the video, but will still click "dislike" just for fun, especially if said video has no dislikes at all yet. They then delight in watching the comment section fall to ruin as a direct result of their interference, with highly intellectual and imaginative responses such as "1 person is gay", or "1 person should jump off a cliff", or "Justin Bieber made a new account to dislike this video" appearing in it.
Furthermore, the less "likes" a video has, the more devastating are the actions of the serial disliker, due to YouTube's like/dislike bar being based on the proportion between likes and dislikes, rather than on actual numbers. 1 dislike shows up as a much bigger chunk of red on the bar for a video with only 3 likes, than for one with 150. In the former case, it's much more noticeable and the commenters' outrage is slightly more justified, though in the latter case, the reactionary comments take on forms not unlike "the dislike bar is the size of Justin Bieber's penis", which is inexcusable.