Sudden serp drop… why?

Basically there is a website of mine that is dropping 1 position day after day… I was #1 first and now I'm #4 after 4 days… It seems every day I'm losing 1 position and honestly this scare me… Over the last month I did no linkbuilding, I have added no article. The KW is easy and "competitors" (they doesn't even target my KW) that outranked me did nothing at all.

The only thing I did over the last week was to restore a PBN that were offline from June and I didn't knew. All my PBNs I…

Sudden serp drop… why?