I think it's the same thing that sometimes redirects hotlinked .jpgs to a galler...

derefr on Oct 9, 2014 | parent | context | favorite | on: Introducing GIFV

I think it's the same thing that sometimes redirects hotlinked .jpgs to a gallery representation of themselves: for anything that looks like a web browser, it checks whether the referrer is the embedding page, and if not, redirects to the embedding page.