It’s not possible for anyone to say with certainty why your ranking has dropped, but my guess would be that it’s to do with your other topic.
Please read Google’s guidelines on link schemes:
Any links intended to manipulate PageRank or a site’s ranking in Google search results may be considered part of a link scheme and a violation of Google’s Webmaster Guidelines. This includes any behavior that manipulates links to your site or outgoing links from your site.