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.

