There's a lot of factors at play behind the scenes. It's possible that whilst you're seeing more traffic, the ads served with the hubs generating your traffic boost are paying less per click and impression. If your higher paying keywords aren't getting much exposure and lower paying keywords are, this drives down CPM because the average across all your hubs is lower even though traffic is at a high.
For example, say that hub A attracts high paying ads with a higher cost per click. Hub B serves ads that have a much lower cost per click. For convenience, say that hub A impressions give you a CPM of $4.00, whilst hub B has a CPM of $1.00. If you had 600:400 impressions of a to b, that earns you $2.80. If you had 400:800 of a to b however, even though you're seeing more traffic overall, the earnings average out lower at $1.60.
I'd suggest you look at your traffic and see if your usual higher traffic hubs have dropped and other hubs are receiving more traffic in their place. This would be consistent with the above scenario and perhaps stem your fears. Traffic fluctuates but it's likely it'll even out again, and income of any amount is better than none.