I study this stuff. Oddly, the answer is probably much simpler than you think.
It would not surprise you to know that spammers are still spandexing search engines and using automated software to find content to use and sites to link to. Once a junk site is created, then bots begin to spider the junk sites and follow links. Get ready, this will happen a lot.
I see at least a dozen new spam sites each and every day. Spammers by nature are hit and run these days. Links can come and go in seconds, minutes, hours, or days. Only a few sites stick around for any period of time. Some use domain tasting (or at least have in the past) designed to test domain names as a tool to get free domain names and then dump them within the prescribed period of time. For those sites that seem to perform well, they actually might get registered.
As well, some registrars are chocked full of junk domain names which are snatched up by the hundreds or thousands and maintained for years. These sites will also come and go almost like flashes in the pan. ENom, Namecheap, and others are notorious for hosting these sites. These investors, as they see themselves, will try and monetize the domain name based upon previous traffic and existing links then relegate them to the dust bin for a while to then trot them out again later. Again, it is the same mechanism of creating an automated site for traffic.
So you say that Google and Bing has stopped this practice?
Yes. You would be right to a point. But those are not the only search engines in the world. Oddly, Russia, Poland, to a lesser extent, China search engines have all become targets. There are others as well. The software will not only create sites, but also automate promotional activities such as link building and search engine submission, etc.
Lastly: One feature sometimes found on these sites are JavaScript bugs that imprint links and advertising. Some sites are designed to walk right up to the line between spam and legitimate site often sticking just one toe into the spam waters and likely will stick around for a while to build specific traffic patterns. These JavaScript bugs will imprint links from a central database that the spammer owns. These links may only exist for seconds or a single page. Lucky you! Your site may be in one of these databases which are as you may have guessed, automated using software.
But let us not forget referrer spam. This is far more rare these days but done for one simple reason. Automated site usage reports. Some create links but few do. But that is okay. Since Google has discounted junk links, of which site usage reports are of special note and understood, the mere mention of a site is still a citation and can count. While there is little value in these mentions, many thousand do add up. And that is precisely the point.
