Below are the few reasons why Google dropped your website rankings:

1. Duplicate content on pages in your website.
2. Changes in Meta tags like title tag, meta description tag also affect your rankings.
3. Any of your current changes done would have had a negative impact like keyword density, keywords stuffing etc..
4. SEO spam techniques.
5. Any of the changes done by your competitor have had a positive impact.
6. Change in Google Algorithms.