This is especially common with a fresh USB port. I learned why recently from the book Open Circuits: The Inner Beauty of Electronic Components. There is a mechanical component that is more stiff before being used, which can lead one to think that they simply had the USB connector the wrong way around.

I highly highly recommend the book by the way, to anyone on HN. Goes all the way from mechanical components to resistors, nixie tubes, CMOS sensors, processors etc. Excellent photography that reveals the internals, along with operation explanations and history for each component.