It's a bidding system on the Adwords side. ( even this is a black box)
Then it's the typical google black box in the middle. No transparency, no disclosure.
Then on the Adsense side, it's quite clear:
Traffic goes up = ecpm/cpc goes down.
Traffic goes down = ecpm/cpc goes up.
I highly doubt that content network advertisers are seeing lower cpc's when this happens. They are likely being charged the same cpc while Page and company skim/steal from the publishers. We can never know for sure the extent of it, because it's the typical black box.
Now if the search rankings or traffic source remain, then like I said, after a month or so, a new higher baseline is calculated and the ecpm/cpc returns to normal. I've been watching it for years. It's not a one time occurrence based on low quality traffic, it's standard operating procedure.