When reviewing its Google Analytics, I saw the domain name was included in the Referral section. And the figure from this domain was more than 3 thousands visitors a day. This case is called Self-Referral.

As far as I know, self-referring often happens in 3 common cases thanks to Google Adwords Support.

1. Tracking code not installed on all of the web pages: If the tracking
   code is not installed on all of your webpages, Google Analytics will not
   be able to track the referral webpage and will assume the previous page to
   be a referral. For example, http://www.domain.com/forum does not have
   the analytics tracking code installed. Visits from this page could be
   construed as referrals.
2. Redirect: Generally, a website may appear as a self-referral if there
   is a redirect. If a website redirects from www.example.com to
   www.domain.com, Google Analytics will record the referral as
   www.example.com. When the user is redirected, the tracking code records
   the referral source as website the user was redirected from. If there is a
   redirect on your website, we suggest you remove the redirect or add the
   redirected URL as the Website URL in your account. Your webmaster or your
   hosting company should be able to assist you in removing the redirect.
3. Frames: Frames may also result in self-referrals. This is because a
   frame in your website may be used to load another webpage.

So if you get the same issue, that is Self-Referral in Google Analytics, please check if you’re in 1 of above cases to have the appropriate solution.