microUSB casing to the ground, ESD protection
Your question is strongly related to the question of how to connect a cable shield/screen to GND and has only little to do with the TVS diode. 

In a „high speed“ transmission standard like USB, the shield/screen of the cable is mainly there as a defence against capacitive coupling of the E-field. This coupling induces currents into the shield, which must drain off somewhere.

Depending on the expected noise frequency and overall device grounding scheme, there can be different strategies to handle this noise current. That‘s why you see different resistors from shield to GND.