There could be different reasons why your keywords dropped; it could be due to:
any changes that were made to the website - changes in the content, also minor changes like title, meta description, headings can influence your rankings,
an algorithm update - in this case, check again the next days, and often you'll see that things have gone back to normal,
any technical issues on the site and load-time - run a site audit to check if everything is fine,
improvements that your competitors made and overtaken you,
SERP layout changes, etc.
We have a great article that dove deep into this that topic and thoroughly explains how Nightwatch helps you identify the reasons and work on fixing it. Click here to read it!