It's probably a USB device that failed enumeration. This could be caused by driver issues, controller bugs, or device problems.
I'm actually currently debugging a curious issue that seems to affect Intel USB 3.0 controllers when using my Corsair Flash Voyager GT (possibly other devices too). It works fine on my ASMedia USB 3.0 controller, but fails to enumerate (with the same unrecognized device message) when connected to the Intel USB 3.0 controller. I see that you're running Windows 8 and the issue I'm debugging appears to be an incompatibility with Microsoft's USB 3 driver (which started shipping in Win8) and Intel's USB 3 controller. Both the ASMedia and the Intel controllers are using the same Microsoft driver but they behave very differently when I log the driver behavior using WPP tracing of the USB stack core components. I've confirmed that this issue affects 2 different machines which are both running Windows 8 and use Intel's 7-series chipsets for USB 3.0. I'm starting to suspect either a driver bug or a controller erratum that wasn't handled by Microsoft's driver.
Can you try to isolate the controller that's generating these hardware events? You can use the "View devices by connection" in device manager to see what's actually enumerating that unknown device.