Suppose a post is sitting at a $100 pending payout and a whale up votes it to $1000 with a single vote. Other whales see that as abusive and place a counter acting down vote restoring it to $100 pending payout. The abusive whale will get the vast majority of the $25 curation rewards on that post.
When it comes to curation rewards the system is currently unbalanced. There is no way to negate the profits of abusive curators."

Link: https://steemit.com/steem/@dantheman/negative-voting-and-steem