Context: Suetonius describes the growing fear that dictator Caesar intends soon to make himself king.
Quae causa coniuratis maturandi fuit destinata negotia, ne assentiri necesse esset.
Translation: This was why the conspirators accelerated their planned efforts [the assassination], in order not to give assent [to crowning Caesar king].
After much thought, I am reading it like this:
Quae fuit causa: this was the cause. I am reading “quae” here as if it were a demonstrative pronoun, a matter which I am very unsure.
maturandi: I read this as a gerund, genitive singular, complement of causa.
destinata negotia: destinata is an adjective, modifying negotia, both neuter, accusative, plural. They comprise together the direct object of the gerund maturandi.
I read this as a case where “gerundival attraction” does not occur. See Morwood, Latin Grammar, p. 109: “Gerundival attraction tends to be avoided . . . by some authors when it would lead to a repetition of first and second declension endings in the genitive plural.”
What is avoided, I suggest, is the phrase “maturandorum destinatorum negotiorum”.
Quae causa coniuratis maturandi fuit destinata negotia
I read quae as neut. pl., “which things,” or “these things”, not as modifying causa. Causa here is a predicate noun, I think.
What seems odd about this sentence is that fuit is singular, agreeing with the predicate noun causa, not with the subject quae.
Allen & Greenough 316b:
b. A verb sometimes agrees in number (and a participle in the verb form in number and gender) with an appositive or predicate noun:—
“amantium īrae amōris integrātiō est ” (Ter. And. 555) , the quarrels of lovers are the renewal of love.
“nōn omnis error stultitia dīcenda est ” (Div. 2.90) , not every error should be called folly.
Corinthus lūmen Graeciae exstīnctum est (cf. Manil. 11), Corinth, the light of Greece, is put out.
Thanks, Qimmik. Your opinion on Quae looks plausible to me. The other part, taking maturandi as a gerund, gave me a lot more trouble. I re-read Morwood on “Gerunds and gerundives”, in which I noticed for the first time the exception that I quoted.