Mine is much more serious. From 48000 to 6000. It was one site-wide link that inflate those YBL, but it's still there...
I don't pay attention to anyone but Google, are your links in google webmaster tools still the same? and you said it was a sitewide link, was it removed or penalized?
I think its a kind of experiment or updation going on, sometimes its restored and sometimes go really low, today my links are restored!
It seems Yahoo siteexplorere is being fluctuated in itself lately. I found the link dropped 50% in some of my website then back to where it was in a next day. I do not think the links was really dropped but the issue with Yahoo siteexplorer itself.