[jffry]

I imagine that there is a nontrivial amount of nobody channels with suddenly popular videos that are part of fraudulent ad revenue streams (if you can drive views at less $ per view than the ads earn you it's a net positive).

This is not to say that all suddenly popular channels are ad fraud. And it seems in character for YouTube to put a blanket constriction instead of doing the trickier job of dealing with separating genuine viral hits from fraudsters.

Or maybe they're working on something more nuanced but rolled out a rough measure to stem an aggressive wave of ad fraud. We'll never know because companies are tight lipped about that sort of thing.