No product is perfect, and the same goes to Google's.
Some ad buyers have been exploiting and manipulating Google's ad auction platform. This has led to a decline in ad coverage and also a drop in fill-rate.
As a result, AdSense publishers noticed a decline in their CPCs over the days. When the search giant realized, the company took action.
Google confirmed the matter, and has addressed the issue by identifying each and every one of the violators, blacklisting them from using its ad system.
The news was first surfaced on WebmasterWorld in August 5th, 2017, talking about the declining AdSense CPCs.
The advertisers were using "irresponsible campaign parameters," said Google in the statement. This enabled them in lowering query coverage for specific creative types in some countries. Among the causes, include the possibilities of those violating advertisers to artificially inflate their click-through rates.
Some publishers thought that this was a glitch. But Google confirmed that it wasn't. CPCs were relatively low at the following days after August 5th, and Google isn't updating them.
But going forward, publishers should see their CPCs back to normal.