A form of pranking. Rules vary by area and participants, but fixie hunting generally refers to a game played by cyclists in which multi-geared bicycle riders "hunt" a metropolitan area for fixed gear riders out on a ride in order to torment or ridicule them. Since fixie riders are generally not as fit as other cyclists and their bikes have a set top speed, they are easy to catch up with, prank, then leave behind.
Activities may vary from a simple demeaning sprint past them, to squirting with water bottles, yelling, spanks on the butt or simply beating the shit out of them. Once said activity has been accomplished, the riders must escape from the now-angered fixie riders and find new fixie riders to torment. Rules vary, but in general most fixie hunters do not stop riding until they find riders to torment.
We went fixie hunting today and pissed off some hipsters.