This could be due to USB-Protocol:
Some non-standard USB devices use the 5 V power supply without participating in a proper USB network which negotiates power draws with the host interface. These are usually referred to as USB decorations. The typical example is a USB-powered keyboard light; fans, mug coolers and heaters, battery chargers, miniature vacuum cleaners, and even miniature lava lamps are available. In most cases, these items contain no digital circuitry, and thus are not Standard compliant USB devices at all. This can theoretically cause problems with some computers, such as drawing too much current and damaging circuitry; prior to the Battery Charging Specification, the USB specification required that devices connect in a low-power mode (100 mA maximum) and communicate their current requirements to the host, which would then permit the device to switch into high-power mode.
Some devices, when plugged into charging ports, draw even more power (10 watts or 2.1 Amps) than the Battery Charging Specification allows. The iPad and MiFi 2200 are two such devices.[62] Barnes & Noble NOOK devices also require a special charger that runs at 1.9 Amps.[63]
Wikipedia
Maybe you has always connecting other devices at the same USB-Port.
Not every physical USB-port is one port on your motherboard.
I recommend not to use your USB-port, as you can easily damage your mb.
Try to measure the output of your phone charger.
