i don't actually know for sure exactly why it was happening, but i have a fuzzy picture of the problem...
quick background - when i setup everything, i used a powered USB hub and configured things so it was only powered when the Jeep was on. my only USB device so far, the OBDII interface, was also set to only be powered when the Jeep was on. however, since the hub was always attached to the tablet (by virtue of it being in the cradle), the tablet would have ended up powering the hub even with the Jeep off. this was undesirable for many reasons, mainly tablet battery drain. therefore, i severed the +5V (red) wire on the tablet OTG cable so that the tablet couldn't provide power to the hub.
i realized recently that a USB device can be considered "connected" with only the 2 data lines intact (i.e. for self powered accessories), so i probably should have used a multi-contact relay to completely disconnect the OTG cable from the hub. i believe the problem was in the timing of everything shutting down, depending on whether the OBDII interface was in the middle of sending data and then maybe even whether the hub or it shut down first (talking micro seconds here).
i happened to have a another very fancy USB hub designed for carputers laying around, the Carnetix P5USB
. besides featuring a true complete disconnect/shut-down that could be triggered by ignition, it also isolates each port and prevents any back-bleed of +5V from an accessory. replacing my previous dumb hub with this one solved the problem. since this hub has so many features, i can't be certain exactly which one was the actual fix.