Increase current draw of a low-voltage DC fan circuit
If this is really the case then put a resistor in parallel with the fan, but I think you have the opposite problem. As far as I understand usb supplies 5V and the only time it shuts off is during an over current condition. For standard and high speed (1.0 through 2.0) USB this is 100mA so that could explain why your port is shutting off if you are plugging it into a PC or a hub that follows the USB spec. 

If you are using out of spec chargers then there is no way to predict what will happen, they are out of spec and shouldn't have the USB label on them. The only way to predict what will happen is a schematic or experimentation. There are no guarantees with these chargers, its the wild west. 

The other problem is most BLDC fans have a wicked load curve and can go from 0mA to full power in one fan cycle, this would wreck havoc on a charger that is expecting a constant current. You'll probably need a cap to smooth the power out.