One way I have seen this done in the past is that you have contest sites. As a user you gain x number of entries for every action you take. One of these actions can be to go in and like or dislike a video for example. Sometimes the sites can even check that the action was performed.

Someone wanting more entries might create lots of fake accounts.

I'm sure there are many other ways to do this but that is one example on how spam dislikes can be generated.