The Yahoo! Search Blog posted a weather report informing us that they are updating their search index.
We are in the process of rolling out some changes to our search results. As usual, you may see some changes in ranking as well as some shuffling of the pages that are included in the index throughout this process. This update will roll out this evening and will be complete very soon.
This must be a small update because I did not find any threads preceding the Yahoo! blog post on this. Typically, SEOs and Webmasters are able to spot the update, before Yahoo! announces it.