"If this is a problem with USB drivers (which are supposed to be loaded in user space, not loaded to the system), why is it causing a BSOD?"
In Windows, drivers (especially in this case) are loaded in the kernel. Kernel drivers are generally just referred to as drivers. Yes, there can be multiple drivers loaded for the same device (it's too long to discuss here, go to microsoft's msdn site and research it), as by stacking, more functionality can be added.
As for WHQL certification, I don't know the detail of such certification, but like most certification, it does NOT guarantee that the driver is perfect. It just says that it conforms to a certain standard. Whether it can f*ck up a system in ways beyond what the certification might try to prevent, is another matter.