J'ignore de quoi tout ceci retourne.

English Translation

I don’t know what that’s all about.

what exactly does ‘retourner’ mean here? I don’t see a obvious figurative definition in wiktionary.

My dictionary has an entry for it:

D impersonal verb
▸ j’aimerais savoir de quoi il retourne
I’d like to know what’s going on

To my mind, the main sense of retourner is turning around something. If you do a complete circle you capture the surrounding area: its environment, that is, everything about it. As in “What’s going on, what’s it all about?”