When your rankings go down, it could be because of several reasons. These are the most common scenarios:
a. Google has released a new algorithm update that might have affected your site.
b. Your website has lost some good backlinks causing your domain authority(DA) to decrease.
c. Your website was penalized/filtered by Google because of violating Google’s guidelines. First check your Google Webmaster Tools account to check if you have errors or messages related to this issue. (You can see this error in manual action of google webmaster tool)
d. Your competitors have improved their SEO and have outranked you.