It seems quite clear that secundus comes from sequundus, a gerundive of sequi. But why did -quu- become -cu-? This change is not universal, since some Latin words do preserve -quu-, at least the end of a word (consider equus and aequus for example). I suppose this sound or spelling change is behind the dative cui and genitive cuius of quis. Is there a rule that could help me predict when a -quu- becomes -cu-? Are there notable examples or exceptions I should be aware of?
Secundus is regular, eqvus isn't
There's a sound change called the "Boukólos Rule", which started back in Proto-Indo-European. When labiovelar consonants (like
/gʷ/) appeared next to
/u/, they dissimilated and lost their labialization, becoming
/g/. The rule is named after one of the first known examples, Greek βουκόλος "cowherd".
In Latin, a u
/u/ vowel continued to turn a preceding labiovelar qv
/kʷ/ into a plain velar c
/k/. So seqv- + -undus became secundus, which was perfectly regular at the time.
The rule eventually ceased to be relevant in most Indo-European languages (for example Greek lost its labiovelars entirely). But in both Latin and Germanic, later sound changes created more
/u/ sounds. And the Boukólos Rule continued to apply to these.
The short o in the second-declension endings shifted to a u between Old and Classical Latin, giving the familiar endings -us and -um. And this new u transformed second-declension nouns ending with labiovelars. This also created the irregularity in the relative pronoun: qvī qvae qvod, but cujus, cui (originally *qvojos, *qvoi).
But while the relative pronouns were fairly fossilized (hence the archaic endings), and the gerundive in -undus was falling out of use, the second-declension endings were still very common and productive. And the paradigm ecus, eqvī, eqvō, ecum, eqvō, eqve looked inconsistent and wrong.
So in writing, it became standard to write qv for a plain velar
/k/, only before the second declension endings -us, -um. Allen's Vox Latina quotes Velius Longius (writing in the second century AD):
Auribus qvidem sufficiebat ut eqvus per unum u scriberetur, ratio tamen duo exigit.
Indeed, to the ears it is fine to write eqvus with a single u, but the mind demands two.
Despite this, forms like ecus are still found in inscriptions, indicating that the true pronunciation was still