For what it's worth, I'm completely certain now that it's the vehicle sensing loops causing the strange cyclometer behavior. I haven't bothered to try very hard to stop it, or establish exactly how the interference is happening, but there are several intersections where I can reliably make this happen by stopping over the loops - and many more where it never happens.

For what it's worth, I'm completely certain now that it's the vehicle sensing loops causing the strange cyclometer behavior. It also seems to be directly interfering with the cyclometer itself, nothing to do with the sensor - I've seen it happen when the sensor wasn't on the bike. There are several intersections where I can reliably make this happen by stopping over the loops - and many more where it never happens - so it seems to have to do with the specifics of the loop, too.