CAT 6 racing or "hipster racing" refers to commuter cyclists who race one another during their commutes to work. As opposed to CAT 5 roadbike racing, CAT 6 racing starts when a stranger riding a bike tries to pass another cyclist and the other cyclist speeds up rather than let them pass. This creates an impromptu "hipster race."
CAT 6 races are often started by testosterone-pumped mountain bikers trying to pass roadies or roadies who are horrified that a girl on a beat down huffy is pasing them going up hill.
Did you see that fucker try to pull a CAT 6 on me? He wouldn't let me pass him on the bridge when I was coming back from lunch.