As a Catholic, I was taught that the general fear was that with ease of access to the Bible in the vernacular, multiple interpretations would sprout as more and more people had the ability to decide for themselves what a particular passage meant, which stood at direct opposition to what the Catholic Church officially taught. Reading the Bible personally I can definitely understand that explanation as several passages if you skimmed through it would seemingly point towards one solution, while the Church through rigorous councils had decided contrary. With the Protestant Reformation that had followed, it was clearly obvious that this would happen regardless and so inevitably the Church had allowed the Bible in the vernacular.

Please take all of this with a grain of salt, as I lack sources and am just referring a personal lesson that could be totally off-mark. I am also grossly skipping over years of Papal conclaves and Councils that would decidedly allow the Bible in the vernacular as we know it.

I think this quote from Pope Innocent III in 1199 regarding the people's reception of a french translation that they could read says something about the heart of their opposition to translations:

In secret meetings the laymen and women dare to discuss such matters between themselves, and to preach to each other: they also reject their community, do not intermingle with similar people, and consider themselves separate from them, and do not align their ears and minds with them; when any of the parish priests wished to censure them concerning these matters, they stood firm before them, trying to argue from the Scriptures that they should not be prohibited in any way from doing these things. Some of them also scorned the simplicity of their priests; and when the Word of Salvation is shown to them by those priests, they grumble in secret that they understand the Word better in their little books and that they can explain it more prudently.