As I’ve pointed out, Google has been making changes both visible and under the hood, at a mad rate.
The latest change people have been able to distinguish is being called “Mayday” because it occurred on or about May 1st – and because it badly affected some websites rankings and traffic.
Drops in traffic as much as 90% have been reported. Other sites have noticed modest improvements in ranking and traffic.
Smaller sites don’t seem to be much affected by this. We have noticed some affects on larger sites and VERY large sites (tens of thousands of pages) can be majorly affected by it.
This article has a good summary and links on the subject.

MAYDAY Google update