Bluetooth devices use a specific radio frequency to talk to each other, a bit like old AM/FM radio channels but at a much higher frequency.

USB 3 devices naturally use cables, however the cables and the devices connected at each end generate electrical noise over a range of radio frequencies. Unfortunately the range of frequencies USB 3 is allowed to use conflicts with the specific frequency Bluetooth devices all use. This means that any USB 3 port, cable or USB 3 device can emit radio frequencies that may cause interference with any Bluetooth devices you are using in your computer.

The end result is that your Bluetooth device/s may degrade in performance, sometimes to the extent that there is very little or no valid communication between the Bluetooth devices and Bluetooth receiver in your computer.

All Bluetooth devices use the same 2.4GHz radio frequency to communicate with each other. The 2.4GHz frequency is also used by most other modern home wireless devices that may be connected to your computer, along with various other electrical items such as hands-free telephones and wireless audio/video cameras and repeaters.

USB 3 devices naturally use cables but the electrical signals passed through them are scrambled at a constantly varying frequency. The noise produced by the devices and cables radiates from them and dissipates in the surrounding space.

Unfortunately the large range of high frequencies that USB 3 is allowed to use includes 2.4GHz, putting it in direct conflict with the Bluetooth signal you may also have in your room. This means that any USB 3 port, cable or device can emit frequencies that will cause interference to a Bluetooth device you are using in your computer.

 If you want to know more and see some nice wave form diagrams you're in luck! Intel have made a white paper on the subject hidden deep within their website that you may want to refer to for a more detailed overview.

All Bluetooth devices (from v1 to the current v4.1 specification) use the same 2.4GHz short-wavelength UHF radio frequency to communicate with each other, as was assigned by the U.S. FCC (Federal Communications Commission) back in 1985. Nowadays many home and office wireless electronics also all use this range due to the limited air space certified for interference free operation.

USB 3 devices naturally use cables but the electrical signals parsed through them are scrambled over a spread spectrum that is constantly alternated by the USB controllers internal regulating clock.

The USB 3 specification requires frequency modulation over a range of frequencies and one part of that range is the 2.4 to 2.5GHz spectrum. A side effect of this frequency range is that USB3 hardware emits low levels of noise at these frequencies into the surrounding environment, putting it in direct conflict with the transmissions of any Bluetooth broadcasting devices in the same space.

 Intel have made a white paper (with downloadable PDF) on the subject. Refer to this for diagrams and more detail into their research conducted in 2012.