While I agree with Crown's point about HurryUp's channel and spamming tags in the description, that is very likely NOT what is going on here, or at least is only a part of it. As evidence, take Vlad and Nikita... a mega-channel that has been hit - one of the highest volume channels on Youtube. You would be hard pressed to claim that their meta-data is spammy or against TOC. One could claim that they are just caught up in broader action against other channels, but that doesn't explain why other kids channels are blowing up.
Although I don't have an immediate example, I would speculate that there are spammy channels that are doing ok.
I'm actually surprised we are not seeing more news about this. If this hit a different large vertical, I can imagine that there would be more coverage. Imagine if Pewdiepie lost 2/3 of his traffic overnight...