Steam Tag Leaderboards uses VigLink to monetize outgoing links when providing links to retailers selling the games. For more information, see this page.
And since you're here, if this seems like something you'd want to use yourself, why not register to VigLink yourself? Yes, that is a referral link. I'll be grateful if you use it, as it will help me keep this site up and running, and most importantly, grow my Steam library. :)