A lot of times, it's for the smallest, pettiest reasons. There are some people who rate properly. But, again, you'll never see that because I don't think they can leave reviews.
But from what I've seen, it's either "the update was too short" or they're lowering your comic to make sure the comic THEY like is in higher rates. This is also a site that has a large number of younger readers (I'm talking 9-year old armies), so the reasons could be just for the hell of it sometimes.
Honestly? I've just learned to ignore it. The rating doesn't really say much? I've seen a LOT of featured comics that have like 5-6 ratings, but they're featured and they have a LOT of comments and likes. Here are some examples. Granted, they ARE finished, but they're still up as featured comics: