Drawing too little current from PC USB
A device on a USB 2.0 port can initially draw 100mA. It then negotiates a power budget and potentially draw up to 500mA.

When you use a USB phone charger, it will simply give you up to 1A.

So the PC is probably giving the port enough power for it to be bright. Then the device fails to negotiate and the PC cuts the power, making it dim. Note that what the PC does and what it is supposed to do may be very different.

There is a lot more information at this question: How to get more than 100mA from a USB port