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.
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.