Many bishops signed VII docuмents because they didn't read them. Just like we accept the terms of use of a software without reading them.
That is not an excuse, however.
There's a wide variety of reasons. Some were foaming-at-the-mouth Modernists that wanted to go even further. Some were just non-theological dummies. You'd be surprised at how many bishops barely had more than Catechism-level knowledge of the faith, and that's why so many
periti were brought in tow with them.
Some spent half their time at "Bar Jonah" and just didn't care. Their idea of being a bishop reduced to building schools and organizing after-school activities (sounds a lot like the neo-SSPX). For them, all this "doctrine" stuff didn't matter, and what mattered were PRACTICAL things like the aforementioned.
Others cared and saw the errors, the so-called
Coetus conservative Fathers. But, when +Lefebvre was asked, he had gotten the impression that they were merely signing onto their having participated in the Council, as +Lefebvre had voted
non placet consistently on a couple docuмents.
But there was in fact the core error that most of them missed ... the heretical Conciliar ecclesiology. Rahner rightly points out that it was by far the most revolutionary aspect of Vatican II and marveled that the conservative Fathers hadn't noticed it or made even a peep about it.
But that's because this notion that pretty much anyone can be saved had already deeply penetrated the minds of even the conservatives. We have statements of Archbishop Lefebvre that non-Catholics can be saved BY the Church (even if not IN the Church). That's identical to Rahner's "Anonymous Christian" thesis.
But once you have non-Catholics being saved, then, since it's dogma that there can be no salvation outside the Church, this means that these non-Catholics have to be IN the Church somehow ... if they can be saved. This, then, is the foundation of the V2 ecclesiology where the "Church of Christ" SUBSISTS in the Catholic Church, i.e. that the Catholic Church constitutes its visible core, but there are others who are either in or "related to" this Church in varying degrees, but inside enough to be saved.