Sudden drops in search engine rankings are usually caused either by a penalty or a change in the search engine's algorithm. Web pages that are deemed to be blatantly spamming the search engines could be penalized, and their rankings will plunge. In some cases, such pages will be removed entirely from the search engine's index. Changes to search engines' algorithms can take place several times a year. During and after this process, some Web pages will see major changes in their rankings as indexed content will be re-evaluated. Unless the page relies on a gray-area optimization method that the search engine is actually targeting through its algorithm change (e.g., link spamming), in most cases, a sudden drop in ranking won't last long as most of the search engines' results will stabilize once the algorithm and resulting page reviews are completed.
