I recently went though my analytics and discovered that 53% of my dislikes happened on a single day. Basically I got in an argument with a couple of guys on reddit, so they found my channel and systematically went through and disliked all of my videos. Must have been pretty triggered to put in that much effort.
I also noticed another spike earlier this month. I called out a guy on r/HiTMAN for creating multiple spam accounts to promote his videos. In the process, I happened to discover he was from India. When I check my analytics, I have six dislikes in a single day on old videos - all of which were from India.