Google confirmed that they pushed out the new Panda algorithm update on August 19th 2012.
Do not panic though as it was only a minor update that will have affected less than 1% of search queries.
Google confirmed this Panda ‘refresh’ by tweeting it.
The main culprit for loss of rankings was the Penguin update which consists of much bigger changes so that is why many websites lost rankings when the Penguin update was done but the Panda update just tidies up the leftovers that Penguin has missed or corrects them if they were wrong.