I'll bet a box of Spudnuts cinnamon bearclaws that the Church, as usual: a day late and a dollar short, defined the canon at Trent because somebody else had come up with a, so-to-speak, counter-canon. So, in the usual reactive mode, they said, "No, that's not it. THIS is it."
I cannot stress enough that the Church does not lay stuff down ex nihilo. (We leave that to God.) She says stuff when an issue of contention arises and she has to rear her substantial bulk up and deliver an opinion.
My daughter just called to complain about a "do" where she worships, at the Mission in San Buenaventura, known to the mapmakers as Ventura, CA. Cardinal Mahoney, the great and terrible, showed up for some anniversary or other. She couldn't understand why they did something THIS inefficient way when THAT way would have worked better.
I reminded her that God didn't choose us because we're smart or good, and out behavior continues to provide ample testimony to that simple principle. The maxim is, "I don't believe in organized religion; I'm a Catholic."
So it is with the canon. Somebody says, "Hey, I think I read in the papers that Luther says 'The Song of the Three Young men' is bogus."
"Did he now? Drat! I suppose we'll have to do something. Somebody go wake up the Pope."
(Actually, that's not entirely fair, Apparently Pope St, Pius V (O.P., by the way) was pretty good, though Protestants might not think so. He wasn't idle, anyway.
Depending on what degree of ex nihilo you mean, and subject to the Bible, I wouldn't necessarily blame her if she did in many cases. Technology is going to force us all into some very new areas soon that are going to be pretty tough. For example, while it is relatively easy to be against human cloning, I would think that the Church is going to have to take some sort of stand when human body part cloning farms become commonplace.