Inherent within current USB standards is a CRC (cyclic redundancy check) on every transmitted packet. Without going too far into the technical side of this, this essentially means that on every packet of data transmitted, there is a check performed that confirms that the data sent by the user was received successfully. In isochronous transmission, which is used for USB audio transmission, when a packet is confirmed via a CRC to have an error, it is dropped from the stream. This means that when there is an error in data transmission over USB to your Amp/DAC/etc., the packet that had the error is dropped and not re-sent. This manifests as, well, a lost bit of data, usually a single sample.
The main factor that drives these errors is electrical noise in the cabling used. Electrical noise in cabling is most frequently introduced as a result of a ground loop. A ground loop most frequently occurs when power and data are sent through the same cable at the same time, as the power communicates through the cable to the device and then back to the source over the grounded shielding of the cable itself. Because of the nature of computer power, there are inherently very small current spikes that occur relatively randomly during power and data transmission. These cause incredibly small voltages between the USB audio device and true ground. Although these are usually in the range of a few thousandths of a volt, microphones and other audio devices only generate a few thousandths of a volt anyway, and they can thus cause the hum, buzz, and general chaos that you hear when this happens.
These issues can be solved using a very simple ferrite bead (also called a ferrite choke). That’s what the little bulbous, cylindrical clip-on at the USB-A male end of a cable is. It collects this high frequency interference, removing it from the chain, and negating the buzz.