Mysterious drop in Yahoo rankings

[abe]

My site, below in sig, has ranked 1st or 2nd in Yahoo the last year for my most important keyword. But recently it dropped to 50th. Haven't made any changes to the site in months.

One other change I noticed is that they dropped the www from my domain name when listing the search results. All the internal pages still show the www ...

Any clues?

[ronmojohny]

Ranking algorithm change a few days ago.

[liljohn]

yahoo tends to drop ppl a lot

[abe]

Yes, I know they did an update recently. But I just don't see the reason for such a large drop. My site is a legit site and very relevant to the keywords, with solid backlinks. I suspect it's the result of some unknown penalty.

Any ideas, just hypothetically speaking, on reasons a decent site might be penalized by Yahoo?