I discovered that it was the Renesas startup app -- nusb3mon.exe -- that was responsible for this.
According to the manual for it, the app is only used in two situations:
1) A USB3.0 capable device is connected to a port on the USB3.0 Host Controller via a USB2.0 hub or USB 2.0 cable.
2) A Lack of bandwidth for Super Speed (SS) periodic transfers (Isochronous and Interrupt) for a port on our Host Controller was detected. This is only for super-speed periodic devices. To produce this situation, multiple super-speed isochronous devices are required. (Currently there are no SS isochronous devices.)