Yahoo! Search is undergoing its monthly update informs the official yahoo Search Blog.
Priyank Garg of Yahoo! Search writes:
"We've been rolling out some changes to our fresh web data and crawling, indexing and ranking algorithms over the last few days. We expect the update will be completed by the weekend. So, as you know, throughout this process you may see some changes in ranking as well as some shuffling of the pages in the index."