Webmasters suspect that Google has pushed out another iteration of the Panda update. There is a discussion about this going on in WebmasterWorld.
Member tedster kicked off the conversation (via Search Engine Roundtable):
I'm hearing some reports of site-wide ranking demotions over the weekend that have a similar pattern to what we saw with Panda. That is, lots of keywords affected, but the demotions are not at all uniform the way we often see in a site-wide penalty.
Has anyone else seen something like this recently? How about (we can hope) any improvement in rankings for a previously Pandalized site?
Some noticed similar changes. Some saw sites previously "pandalized" rise, and some reported that they saw no changes.
One member responded that they had "sustained dip on sites with a local relevance, beginning on the 17th," and "sustained 40%-50% increase on niche-info type sites, started on the 13th."
It's unconfirmed that this was actually Panda, but it may very well have been. Barry Scwhartz is already calling it Panda 2. 5. He lists the dates of previous iterations, and they seem to come about once a month, so it would appear that a September version would be about due.