Some people follow downvote trails. Sometimes there are certain tags they downvote just a tiny bit to express something. I'm not sure the logic or reason behind each user. This downvote doesn't affect your reputation or anything. It seems to me they just want to express themselves. If you have time you can research more about the user and usually find out what is up. Sometimes there is an interesting story like an account was hacked... or ... you find out and it may be a good enough story to write about.