Wallace Shawn as Vizzini in the movie "The Princess Bride" gives the best explanation ever given of how Google re-ranks sites when it decides a new algorithm is needed after deciding a new way of looking at things is needed:
So Google re-ranks sites when it is using a new algorithm to try and figure out what matters and what doesn't by going back and forth between it's changing views of different sites until it settles on the truth.
Google has rolled out some dramatically new algorithms to better rank Twitter user pages because Twitter is too new for Google to properly understand how it fits in on the web. This causes Twitter user pages to go up and down in their Google Toolbar rank for some time. One of our Twitter sites' ranking has gone 2;4;6;4;6;2 (and isn't done yet, based on the rankings of similar pages) and one of the Twitter founders' pages has gone as low as 0 in the bouncing.
When Google uses a new or newer algorithm to rank sites (which they do all the time) ranks have to go up and down for awhile. In a broad sense, everything on the web links to everything else, so as you re-rank the first sites in a chain, it affects the next sites in the chain, and so forth. The interesting part is when the rankings of the chain come back to affect the first site, and that site's rank can suddenly change dramatically.