The power lead in a USB cable is a piece of wire that can take power either direction. It is not really a problem or a symptom, it is just an interesting condition. Now that USB supplies power to many devices, there is one more set of things to consider.
Well my understanding of USB power is that it is derived from the PC's 5vdc power supply and usually has some kind of overcurrent detection/protection. I can't see any means where the USB power wire could 'sink' current. If you could link to such information that would be great, otherwise I will chalk it up to misunderstanding or misuse of terms.