On a Search Engine Watch Forums thread, AussieWebmaster asks if any advertisers have noticed an increase in impressions but a decrease in conversions. He believes that there is some "serious" expanded matching going on, especially as he found this tidbit at Yahoo's Search Marketing Help Section:
The Advanced match type displays your ads for a broader range of searches relevant to your keywords, titles and descriptions and/or web content. If you use the Advanced match type, and to help maximize the relevancy of your listings to search users, make sure you take advantage of Excluded Words, which are words or phrases that prevent a listing from matching a search query.
It's possible, some say, but the other possibility is that the Yahoo! network has added new partners.
Mel66 believes the problem is a combination of the two:
I think you're both correct - Advanced Match is too expanded for my liking; and you definitely have to watch for low-quality partners coming in and out of the network. We had one pop up a few weeks ago that sent us hundreds of bad clicks in a few days. By the time I contacted Yahoo about them, they'd shut down their site.
Forum discussion at Search Engine Watch Forums.