Yahoo! is about to begin a search index update which will re-index content on the web, it will result in ranking changes and “page shuffling”.
The Yahoo! Search blog reports: We are re-indexing content on the web to launch this change, and you should immediately begin to see the changes on Yahoo! US, Yahoo! Japan and Yahoo! Korea. (Other regions will roll-out in the future.) This will be accompanied by the usual ranking changes and page shuffling that is typical of weather updates.