Google reportedly handed out a bunch of penalties over the weekend for unnatural outbound links. These are unnatural links going from your site to others.
According to Search Engine Land, many of these were issued over the weekend, but it's unclear if there were many for unnatural inbound links (as in the sites being linked to by the links in question).
While Google hasn't issued a statement, a lot of webmasters have reportedly received the messages in Search Console.
In the Google Search Console help center, Google displays this video discussing what it means by unnatural links from your site:
The video is nearly three years old, so presumably not much has changed here in terms of how Google defines this penalty. On the same page, Google has this to say about it:
If you see this message on the Manual Actions page, it means that Google has detected a pattern of unnatural artificial, deceptive, or manipulative outbound links. Buying links or participating in link schemes in order to manipulate PageRank is a violation of Google's Webmaster Guidelines.
As a result, Google has applied a manual spam action to the affected portions of your site. Actions that affect your whole site are listed under Site-wide matches. Actions that affect only part of your site and/or some incoming links to your site are listed under Partial matches.
Over at Search Engine Roundtable, there are six links to different threads in Google's forums with people complaining about the penalty.
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