Does it do that even if you open it to a completely empty directory and leave things there?
For the best test, create an empty folder on your desktop, quit Opus and make sure dopus.exe isn't running, then right-click the empty folder and choose Open in Directory Opus to load Opus showing that folder, without any other folders being loaded. (If your default lister has other tabs which open on startup, get rid of them, re-save the default lister, then exist dopus.exe and open the folder. That way the other folders/tabs are ruled them out, and only that empty folder is ever loaded.)
Another thing to try: Set Preferences / Miscellaneous / Advanced: no_external_change_notify to TRUE and see if it still happens. (You'll want to set it back to FALSE after doing the test, though, since it will stop Opus noticing changes to folders.)