Since we're speculating, I suggest it has something to do with "selfies" originally. Or maybe it started with the sense "just among us ladies" and then got generalized to mean "far from the madding crowd. BDAG lists it, as does BDF, as idiomatic, even written as one word καταμόνας, like καθόλου.
Carl W. Conrad
Department of Classics, Washington University (Retired)