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

There are a number of accounts going around downvoting everything at the moment. I have received some flags myself. According to the article below all these accounts are the work of one person:
https://steemit.com/flag/@guiltyparties/the-flag-ring

The good news is that:

• Their SP and voting power is too small to have any financial effect on your posts
• Their account reputations are lower than yours so their flags don't affect your reputation
• Steemcleaners are gradually cleaning them up by removing their delegated Steem (the Steem account provides the initial 15SP to all new external accounts but this delegation can be removed)
• You didn't do anything wrong, so there's nothing to worry about.

The easiest way to check if you are being flagged by one of these accounts is to look at Steemd. Here's your flagger:
https://steemd.com/@fertipo

• 15 SP
• Voting power: 20%
• Nothing but downvotes

It's not something to worry about.