Weird. On Friday afternoon, I downloaded the latest Overdrive update. Prior to that, Overdrive was a mess and functioned so horribly I'd uninstalled it. I couldn't download files often and even when autodownload was disabled, it had set itself as the default for downloading all epub and pdf files. Since I downloaded the latest version, however, it's been fantastic.

I can't explain the difference in behavior. Maybe one of the other posters can.