Google makes small changes to its algorithm daily and also very few months, when they push their page rank, they push a bigger algorithm change. Last major one happed end of April which many SEOs called it Google Mayday Update. But that was not last major change by Google and will not be the last.
Sudden PR and keyword ranking changes, dropper and losses, are usually result of these big changes however growing Google filters, designed to prevent link spams, continue to devalue incoming links, and in many cases also penalize sites for artificial link spam trying to tweak Google rankings.
As an experienced SEO expert in Los Angeles, I have many clients in Los Angeles area, Orange county, San Diego, and Southern California that I visit face to face. They are mainly small businesses owners who think they can jump on top of Search engine SERP on their major keyword so they they hire some so called SEO specialist from overseas without checking his or her work or credentials, and when they don’t see any progress in their ranking, and in most cases drop in ranking and some even see a penalty, they look for local SEO consultants in the area to help them.
Of course local SEO experts aren’t cheap and we don’t promise overnight result. If you are a small business owner and you think your site has been penalized by Google or you have had a huge keyword ranking, before contract anyone make sure you do some research online and see what you can do yourself before hiring an SEO specialist. There are a couple of articles I recommend:
Here is one by Google
http://www.google.com/support/forum/p/Webmasters/thread?tid=3ba4e344e391299e&hl=en
And here are a few more
http://www.ksl-consulting.co.uk/google_penalty.html
http://www.searchenginegenie.com/Faq/content/4/267/en/how-to-remove-google-penalty.html
http://seo2.0.onreact.com/google-filters-exact-match-anchor-text-links-are-the-new-meta-keywords