There are numerous theories abounding. Most are way off track. Essentially, what google has done was employ a filtering program to weed out over-optimized websites. Now, their criteria for over-optimzation is what is coming under the gun.
Personally, I don't like the filter. I find myself using altavista or alltheweb to perform my searches.
The irony is that my clients are doing even better now after the filter than before it. However, as a user of google, I wish the would undo it.