It was a gradual discovery of Mgr. Viganò. For long he believed (and hoped) that the Church would have a self correcting and purifying power without the necessity to bring the evil out. The McCarrick case - and the role of pope Francis in this - made him aware that the Church was so corrupted that only exposing of the evil is the way to fight it.
An interesting perspective, for sure. I'm not sure I'd want him to be silent, because what he expresses is like a bolt of lightning in the darkness and most in the Church WANT him silent, but he had to be aware of certain things going on for quite a lengthy time period.