Without checking your website it is impossible to check the reasons . But still i tell you some following reasons that may impact on off page seo and traffic downfall.
1.Lot of 404 errors . Check it google crawl errors .
Your average content quality is lower . Suppose your website have 10 articles . 3 articles have lots of traffic ,less bounce rate , high page view time . But the other 7 have low page spend time , high bouce rate . In that case your overall website quality dropped and traffic also dropped . To sove this problem Remove the low performing content from website or modify it . And use 410 redirect so search engine can also remove it . If you don't know how to remove content properly you can read this .
One more reason that imacted to lots of website that is create lots of low quality backlinks . Always choose no backlink rather than bad one . Moz also recommend to remove bad backlinkg . Learn how to find bad backlinks