I got huge down in reffering page

I got huge down in reffering page, it result down in seo. What is the reason of this

Here is graph which i got from ahref. you can see in last 15 days i got huge fall.
http://i.gyazo.com/51377eb8244e9d9cd3a3ed1cb2cb0961.png

Please tell me possible reasons of this