Thursday evening the Yahoo Blog gave warning of changes to their algorithm.
“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.”
Remember, do not react rashly to any short term changes. Updates tend to take a little time to stabilize so keep that in mind. All the best in the coming weather!