Eighteen '1' votes from twenty four users in one day???

In last several months average rating of the movie was slightly increasing almost every day. It rose from 5.38 to 5.50 and every days when I was looking at this page from two too six or seven users voted for the movie.
But today 24 users voted for Free Willy and 18 of them rated it "1". Since May of last year only 40 users rated it "1" and among them 18 voted today. Imagine that: in eight months 22 among 880 and in one day 18 among 24. Immediately it caysed serious decreasing of weighted average rating. I bet someone has been working very hard to make such votes and rating changes. I don't have any idea what is the point of these votes and don't understand why weighted average rating, which is supposed to protect system against such people also decreased so strongly.