Its referral spoofing, this is something that has become more and more popular over the last 12 months from theiving webmasters trying to gain access to your members area feeds by putting in referring URLs that make no sense whatsoever.

I found a site a few weeks ago that was doing this, for some reason to access paid feeds we dont even have LOL

It could also be that they are sending traffic from sites with illegal content on them and they are spoofing the referral URL to hide this fact from you, the best way i have found to combat this is just to de-active the affiliates account until they get in touch, sooner or later they will contact you to find out why they arent making any sales from their sites full of stolen content $0.02

Regards,

Lee