"Google executives blamed the decline in search ad rates on forex fluctuations and ad format changes, but analysts wondered whether mobile advertising -- which has lower rates -- played a more important role than the company admitted."
http://m.yahoo.com/w/news_america/google-results-fall-short-rare-miss-hurts-shares-001214475.html
It also probably doesn't help that Google are on a crusade to reduce the visibility of small web publishers who use Adsense, in favour of large brand sites that do not, or use it in a limited way.
Could it be as simple as everyone is tightening their budgets? That seems more likely than any, IMO.
Besides the points above I think some of it is that people are starting to realize that mobile traffic doesn't convert in the same way as desktop, just like how most figured out how content/social ads have a much lower conversion rate. Also, not really being talked about much, is their hardline stance against PPC affiliates.
We're reasonably involved in adsense through a couple of projects. Couple of observations...
1. CPC down in most sectors
2. CPC still rising in sectors where they are just cottoning on to google search
3. Gap between content + search network CPC increasing - presumable more people turning content ads off
Mobile isn't currently a big issue for us. The biggest tip I could give is to try to get on AFS.