If you delete the “/_security_check/” part from the URL, you get to the site proper. Maybe it's a reaction to a steep surge in requests. Edit: Or, as another

If you delete the “/_security_check/” part from the URL, you get to the site proper. Maybe it's a reaction to a steep surge in requests. Edit: Or, as another comment proposes, it could in fact be due to the referrer, and we're being had.