This picture was posted on a 400 plus page thread of Killboy.com 'fail' photos, mostly showing really bad riding form, crashes and squids.
At first glance, looks like the poor guy had a minor crash and was still riding. No gear except the helmet but the bike still looks ok so maybe it was a slow speed get off or something.
Then someone else posted this thread.
The guy crashed, but before that, it looks like he was purposely dragging his knee....wearing only jeans.