Strange. It's not on by default.
If you haven't changed much in your configuration, doing a config reset may be the easiest thing. (Settings > Backup & Restore > Reset, from the main menus).
You have to expand the different sections of the list to see what's in them.
I would also turn off "Automatic Formats" on the page below the Folder Formats in case those are what's doing it. (But that would only happen if you turned on grouping in a folder and Opus remembered it. It wouldn't happen by itself.)
Another option is to reset all your folder formats, which you can do via Settings > Set Lister Defaults from the main menus, and then choose the relevant option in the dialog that opens. (Or see Folder Formats: Quick Guide for another way.)