A quick update from the Yahoo! Search Blog;
We’re in the process of rolling out some changes to our crawling, indexing and ranking algorithms. While we expect the update will be completed soon, as you know, throughout this process you may see some ranking changes and page shuffling in the index. To share your thoughts on this with other Yahoo! Search users, please visit the Site Explorer Suggestion Board.
Update: With this weather update, you might also experience spikes or drops in crawl traffic for the next week or two.