Adding those grounding wires from my desk lamp and the aluminum plate under the keyboard / trackballs to the PC case reduced the problem, but didn’t eliminate it.
Logging all that data, though, pointed to what (I think) is the cause: static discharge. I’ve been touching the screws on the wall switch before sitting down, which pretty much made the problem Go Away. Touching the (now-grounded) desk lamp or the keyboard plate still kills the hub, so the hub inside the PC must be way sensitive.
The disconnect follows the external hub’s cable, which means (I think) the jolt’s entering through that wire. I’ve already tried different cables, but perhaps different routing will help; there’s a huge tangle of wires behind the desk.
It’s enough to make one swear off this USB stuff!