The police are still investigating WHY this happened. What we know is that a group of people on motorcycles in NYC chased down an SUV. The bikers beat the driver and slashed his face. According to Gawker:

Yesterday afternoon, for reason’s that aren’t clear, a large motorcycle gang surrounded a black Range Rover on New York City’s West Side Highway, forcing the SUV to stop. The SUV’s driver, who was reportedly traveling with his wife and five-year-old daughter, runs over two of the bikes and speeds off. The bikes give chase — one of the motorcyclist’s helmet cam captures the entire chase — and follow the Range Rover as it eventually exits into Manhattan. At a stoplight, the gang again attempts to get to the driver, pulling open his door, before the SUV drives away. The gang catches up to the SUV at another stoplight, at which point one rider takes off his helmet and smashes in the Range Rover’s passenger window. Then the camera cuts off. Range Rover’s passenger window. Then the camera cuts off.

The driver was treated at a hospital and was released. The video is 6+ minutes of the bikes chasing down the SUV. If you want to see the window getting smashed in, skip to the last few seconds.