This is a weird one. If your computer's USB port provides dirty 5V, the onboard DC-DC converter tries to compensate, drawing more current. The service manual reveals the isolated ground plane layout. A "hot" fix from the manual: Cut the USB shield trace (JP3) and install a ferrite bead.
Behringer typically provides user manuals publicly, but full component-level service manuals (schematics) are often restricted to authorized service centers. To find the most recent diagrams: Visit the Support Portal. Check reputable technician forums like GroupDIY or BadCaps . behringer xenyx qx1202usb service manual hot