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Hi there. My thought is that it was just a coincidence. Usually, responsiveness doesn't affect rankings that dramatically, especially on desktop devices. There are many-many far more important ranking signals than being mobile-friendly. Make sure that your website is properly optimized, you got good backlink profile etc. Also I suggest to look at your competitors. If they have improved, or there were new companies, which are better optimized than yours, and, at the same time your website is underoptimized - your rankings will go down. Hope this helps.