Yes, I think I know what it means here. I think the author is using the word "subjunctive" as the adjective of the verb "subjoin" just as "adjunctive" is the adjective of "adjoin" and "conjunctive" is the adjective of "adjoin" and "disjunctive" is the adjective of "disjoin" and "injunctive" is the adjective of "enjoin". I have never heard anyone use "subjunctive" as the adjective of "subjoin", however. "To subjoin" something means "to append something at the end" or "to add or attach at the end of something spoken or written, etc." This is the verb whence we get the word "subjunctive."
Anyway, I think, in this instance, the author uses it figuratively to mean that the "recriminations" or "counter-accusations" are being added at the end, i.e. that somebody had made some accusations and then, in the end, a torrent of these "recriminations" came out of nowhere to be levelled against him. That's my guess; it's an educated one, but I have no proof that that is what he's using it to mean because no dictionary that I could find has a definition of "subjunctive" as "of or relating to being subjoined"; I am just following the paradigm of other etymologically-similar words.
I hope that might have helped answer your question, nonetheless. Take care and good luck in your studies.