There are many website who spam your website URL so that you see them as your referrer and the reason to do this can be anyone of the following

1. You have log opened to public and so there url as referrer helps them get a backlink.
2. You may need to sign up as why he is referring you.

Mainly this is done by admin-shop and there partners site.
and you can block them by using the .htaccess of your web site abd putting the following lines in your .htaccess.
Code:
RewriteEngine On
 RewriteCond %{HTTP_REFERER} ^http://(www\.)?domain1.com/.*$ [NC,OR]
 RewriteCond %{HTTP_REFERER} ^http://(www\.)?domain2.com/.*$ [NC]
 RewriteCond %{REQUEST_URI} ^/.*$
 RewriteRule ^.* - [F]
Thanks
Shabbir Bhimani
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