OK, I changed my robots.txt so that there are leading slashes in front of each page (previously, there were only leading slashes in front of vdir's).
See the attached "robotsneedroot.png" for a graph of my bandwith usage.
- Late Oct - bots appear to have changed their behavior!
- Nov - I had to pay for extra bandwidth this month
- Early Dec - I made the changes to robots.txt. Shortly after,
- PROBLEM SOLVED
More details at my bug report