many possible explanations 1. a high bidding advertiser (s) pulled out of the advertising network or lowered bid 2. random ad rotation 3. change in content to trigger lower paying ads 4. advertiser maxed out his budget
Great explanations of possible reasons. You obviously know what you are talking about in regards to YPN. I was about to type this all out until I found that you already had.
I've been getting more clicks recently, but then all of a sudden the CPC was like 1/3 what it used to.