Why there's a lot of downvoting happening on steemit on the past few days even though posts didn't violated steemit's policy?

I got 1 downvote a few days ago and I reviewed my post if I violated any policy. Then I saw a post from another user telling that she also got downvotes and even her friends got downvotes even as far as they know they didn't violated any policy. I hope someone could answer this.