*Near unanimous*? So it’s not the pope himself who decides it but he gets to make the pronouncement?
If the Holy Spirit is leading them, it had better be unanimous.
So how does this happen?
Does the pope get up there and proclaim (something to the effect of) Thus saith the Lord?
Does he get a *feeling*? Sit in a certain chair? Have someone else tell him? Make the decision on his own that it ought to be ex cathedra? Or is he just a figurehead who gets the privilege of making the pronouncement that others decided?
And how do they decide? VOTE on it?
The Pope can issue what he wishes to issue if he so decides. But the episcopate is a collegial institution and the Pope often will (and did in the two instances I referenced) work in union with the other bishops.
If the Holy Spirit is leading them, it had better be unanimous.
I'll bookmark this comment for later reference when you or someone else says that "Bible Christians" require no Magisterium because the Holy Spirit leads them. I trust you'll apply the same standard of unanimity then, right?
Iirc, there was unanimity on the doctrines themselves, just a few voices (John Henry Newman being one notable) who cautioned against issuing the formal definition at that particular point in time.
So how does this happen?
There are books and other resources out there if you're genuinely interested in knowing the times and circumstances that have led to the Church making formal definitions of doctrine. For some topics, the pithy 2-sentence answer you seem to expect isn't possible. So how does this happen?