Over the weekend Google slapped mass penalties for unnatural outbound links.
Receiving this message means your website is linking outward to other websites in a way which is not as per Google guidelines.
John Mueller commented, saying the penalty is related to the warnings over free product reviews that are not nofollowed.
He said, “It looks like you're getting some good advice here already (thanks, everyone!). Travler's link - https://webmasters.googleblog.com/2016/03/best-practices-for-bloggers-reviewing.html - is one you should really be going through (in addition to the more general webmaster guidelines). There's absolutely no need to nofollow every link on your site! However, those that are there because of an exchange (such as a product or service for a review) should have a rel=nofollow on links to the product, to their sales pages, and to any social media profiles that are linked because of the review. Also, it's always a good idea to clearly label these kinds of posts for your readers too.
Once you've cleaned up links like those, feel free to submit a reconsideration request through Search Console, so that the webspam team can review your changes and remove the manual action. More on that at https://support.google.com/webmasters/answer/35843”Google Penalizing Websites For Outbound Links Over Free Product Reviews!,