It was because they were told by the Church (through her visible and ordained priest) that everything they had believed was WRONG.
And they weren't told gradually, encouraged and given options, as BXVI has most graciously done in trying to undo the damage. It all happened overnight, without any warning, without any choice, without any refuge.
They were shaken to their core, and they figured if everything they had been taught was wrong, why stay?
It sounds very distressing. I’m glad I wasn’t born then.