I admit that it's a little rude to just yell "get a light" at a fellow cyclist, but the sentiment is completely legitimate. Riding a bike in the dark is dangerous enough as it is, let alone if you're lacking some sort of illumination. It's hard for other cyclists to see you, as well as motorists. Seriously, it's really in your best interest to get a damn light (which aren't necessarily all that expensive) if you're going to ride at night, so I do understand where O'Brien was coming from.
Two bicyclists came up behind O'Brien, with one saying they were going to pass him on the left. As they passed, O'Brien said, "Get a light."
Dunlavy apparently asked him to repeat himself so he did, with the couple then telling O'Brien to mind his own business. Dunlavy apparently then tried to run O'Brien off the road.
The pair followed O'Brien to his home, where the light talk continued.
The female said it appeared O'Brien had plenty of lights and asked for one, so he gave one to her, but Dunlavy still was upset and clamped his hands around O'Brien's head, according to a police report.
The report added that he twisted O'Brien to the ground and kicked him in the ribs, but Dunlavy denied doing so when he was arrested.
Regardless of whether or not the statement was rude, there's absolutely no reason to then harass the guy for it, going so far as to follow him home and smack him around a little. Cripes, O'Brien even gave them a light for free.
But I guess some folks have anger management problems, or maybe just major insecurity at being called out in front of their girlfriends. Who knows.
Point is twofold: night riding requires a light, and really, we all need to chill the fuck out.
EDIT TO ADD: As always, Bike Snob NYC provides some humor on the situation.