I think a lot of people just go around disliking videos haha
It might sound dumb but maybe they have left over credits on those botting sites and think "hey why not just make someone sad"
I don't mind dislikes that much, unless of course its like 10 or more D:
I get some dislikes here and there but someone once told me not to get hung up on that because dislikes can be the same way as likes are seen. It's all interaction with your video, which is always a good thing