As Evile and Pesetsky show in a newly published paper, “whom of which” obeys very specific rules, whose nature contributes to a larger discussion about sentence construction. The paper, “Wh-which relatives and the existence of pied piping,” appears this month in the journal Glossa.
“It seems to be brand new, and it’s very colloquial, but it’s extremely law-governed,” says Pesetsky, the Ferrari P. Ward Professor of Modern Languages and Linguistics at MIT.
Direct link to the paper.
Two things come to my mind:
1 - hypercorrection. The phenomenon is mostly constructions with "whom", a word that is prescribed as correct but has fallen into disuse among native speakers. I think that this is giving it a bit more room to adopt syntactical roles that it wouldn't otherwise
2 - Romance wh-word (or rather q-word, same thing) chaining. I'll illustrate it with Portuguese due to familiarity:
This sort of chaining pops up in a few other Romance languages too; for example, French. In Portuguese at least it required a copula verb "gluing" both q-words, but this verb is often omitted nowadays, when using this construction; and I think that the same is happening with "of" in English "whom of which". As such, it might be useful to analyse those "whom which" sentences as still having an implicit R(of).