When connecting or disconnecting wires, one of the ground signals from the host adapter is accidentally brushed against a power supply pin on the test circuit, such as +5V. If there are no other ground paths between the test circuit and the computer, nothing will happen. However, if there is a ground path, then current will flow from that voltage supply through the host adapter's ground pin, through the USB cable and the host PC, and then through the secondary ground connection—either MAIN earth ground or another USB port, back to the ground on the test circuit. Basically, that is the same as shorting out the voltage supply on your test circuit, but it uses the host adapter and your host PC as the short circuit, which could damage all components in the loop.