If you have a high-volume keyword that has dropped in Quality Score, you'll see an alert on the Dashboard (Audit alerts/Keywords). You can click the View affected keywords button to see the details. 

Now you will be able to analyze all the keywords that have dropped and see which Quality Score subfactors have led to the drop. Click on the icon in the Quality Score History column to plot the historical values of all the Quality Score components and use the data from Expected CTR, Ad Relevance, and Landing Page Experience columns to identify what you can do to improve the results.