Attenuate and disperse ESD change on USB connector shield
Usually the ESD test will find it's way to the USB shield and the signal lines are usually also protected.
Shield should be connected to GND through a resistor (1M) and a parallel capacitor (100n + 1n). You can add other connector shield or the case to device GND using the same connection point. Some people will disagree, but this solution has been tested many times functionally and in EMC/ESD tests and it's also recommended by USB chip designers.
You should also have TVS diode (datasheets will state if the diode is good for your USB protocol) on the signal and V_BUS -lines and series resistors, maybe even a common mode choke on the signal lines.