Yahoo! has announced that it would be rolling out its 'Weather Update', that would include changes in the crawling, indexing and ranking algorithms. The expected update will continue for a couple of days, but Yahoo! expects to finish it soon.
As expected, there might be some unusual ranking changes and page shuffling in the index, but Yahoo! Hopes to finish the update as soon as possible and that should surely keep the confusion and concerns among the Webmasters to a bare minimum.
According to Yahoo!, "As you know, throughout this process you may see some ranking changes and page shuffling in the index."
In May 2008, Yahoo! had rolled out its search index update and as expected there were some bizarre ranking fluctuations.
Therefore, if you witness any unusual ranking changes, do not get all worked up. It's just the Yahoo! update.