According to the WebmasterWorld, it has been reported that Yahoo! Search is undergoing an update.
To confirm this statement, let me tell you that a lot of web masters have noticed various changes in the Yahoo Search results. As such there has been no confirmation from Yahoo!
It has also been reported that Yahoo Search is undergoing an algorithmic update as well. This update would be followed by variations and fluctuations in search positions as well. (As observed by the WebmasterWorld )