I believe this is actually a computer glitch, not a DAC limitation.
The USB standard states that any USB device should be activated and recognized by the computer at the moment it is powered up. From that moment on the digital output of your computer will be directed by the operating system to the (then) available I/O unit: the USB port.
In order to de-activate the USB port one must power down or eject the USB device.
Windows usually has a small Eject icon on the lower right hand corner of the Task bar.
Mac OS requires one to find the device on Finder (duh) and eject it.
I hope this helps