I got a few months old website that one of my highly competitive keywords got 2nd page of google and working its way up, and then suddenly a few weeks back....this keywords were nowhere to be found in google! At least I still got other long tail keywords with lower competition on frontpage, but these keywords' SEOT can never compare with my primary keywords that disappeared....anyhow, google is changing its way everyday and I would suggest any webmaster who depend on google for a living: Either get yourself on the top of the game or find other sources of income that is totally non-google related!!
it happens regularly but the truth is that google is indexing new pages every few seconds and unless you work hard at remaining competitive then you simply will drop out of the listings. You need o keep building fresh content on your site, regularly updating you current content and also building backlinks. All the best