Wake on USB event

One of my workstations refuses to stay asleep.  Often,
just after being COMMANDED to sleep, it would reawaken.
Sometimes, a short while later (i.e., just long enough
for me to leave the room and return to find it running,
again).
Windows logs suck.  But, I was able to trace the cause
of the wake-up to "USB root hub" (I think there are
six things with this name in the device tree!).
I narrowed it down to the keyboard; unplug keyboard,
command it to sleep and it stays asleep.
[N.B. The keyboard works completely as expected when powered on!]
Fine.  Replace keyboard, problem solved.
*BUT*, I can't EASILY get any more of this particular model
of keyboard (eBay-roulette?) so I would like to salvage it
given that the "problem" is likely a hardware issue.
First question is what *might* (likely) Windows be seeing that
makes it think that a valid USB event has been detected?
I.e., is it a "smart" pattern recognizer (like a WoL packet)?
Or, just "any activity" on the data lines to the i/f?
It seems that power conditioning on the keyboard should be a good
starting place (it's ALWAYS power-related!).
It also seems like Windows only leaves USB ports powered up
during sleep *if* they were active while the machine was awake (?).