Interestingly enough, it turns out if you take a few seconds to search on Google for Google Phantom update
you'll find a whole bunch of articles that explain and analyze the situation in great detail.
In a nutshell, for those who can't be bothered to do basic research on their own:
Around May 8, there appears to have been a significant update in the Google algorithm. Unlike other "named" updates (such as Panda or Penguin), this update was not announced or officially acknowledged by Google.
As a result it was dubbed the "Phantom" update.
The change appears to primarily affect sites that have thin content, but it also appears to impact sites with extensive cross linking among company-owned domains. SEOs are reporting some clients with up to a 45% loss in traffic (most I've seem to be in the 20-25% range, though).
Obviously, it didn't affect all sites. And since SEO is a zero-sum game, any site that lost position has to have been offset by another one that gained. I haven't seen estimates of how many sites might have been hit, but (as usual) the ones who saw a loss are making much more noise than the ones who gained.