If you don't know what user experience signals are and you want to get good ranking then take a look at what Brian Dean and Larry Kim came up with. There are some serious suggestions here that it could be poor CTR that cause google to drop your site rank.
User Experience Signals is a metric that can now be accurately measured by at least google. For example from clicking on a search result how long did the user hang around on the page and the site that was linked to ? Did they BOUNCE that means leave right away - find out if you have some of these pages and eliminate them or improve them so people stop leaving. How to find them and fix them is in the article and infographic on User Experience Signals.
It is google finally able to be forcing the quality of websites on their search engines. Ask yourself how good have google search results been for me and did I stay with a decent dwel time ? If your site is not mobile friendly will this cause a low CTR ? It may well do and within google analytics you should be able to measure theses stat more effectively.