Seems to have stopped now. Not sure what changed, except for Steam having been restarted.
It's happened again. First SteamVR shutdown after a Steam client update. (and a while playing HL:A).
I briefly switched away from X (to ensure that the defunct DRM lease was cleaned up; should SteamVR wait for that?) then re-started and exited SteamVR without further incident.
Nothing extracted yet.
Happens every time on exiting SteamVR in the normal way. Seems to be harmless; looks like one which should be ignored. Started happening after updating to 1.11.4.