Recently some changes were made to the Steem blockchain that allow people to do about 2 or 3 downvotes a day without using the Voting Power that is better spent on upvotes. The main reason is to prevent abuse. I wouldn't worry about getting small downvotes worth less than $0.01, the people doing that are just trolling. Chances are your post is worth so much anyway, the net effect of these small downvotes is nil. If you actually do something considered 'abuse', the backlash will be severe and obvious.