This update “noticeably affects only ~1% of queries worldwide,” said Google on Twitter.
There were earlier rumors of an update over the weekend but Google said the rollout started today and not over the weekend.
The previous Panda update was on June 8th and before that on April 26th. Typically, Google pushes out algorithm updates for Panda and Penguin every month or so. While the last Panda update was just over 2 weeks ago, Google felt they wanted to push out a new refresh.
Here is the tweet:
Heads up: we're pushing a new Panda data refresh that noticeably affects only ~1% of queries worldwide. More context: goo.gl/HNvCt
— A Googler (@google) June 25, 2012
Google said there were no updates to the algorithm or changes in the signals. This was simply a basic data refresh where they ran the algorithm again.
For more on Panda update, see this article.