Keyword dropped ?

It might be a Google dance where keyword ranking keeps on changing for a couple of days. Try to analyze your competitors and check with their rankings as well. If there is no change in their rankings then you will need to work hard on improving the number of quality backlinks and as well as check if you need to work again on onpage optimization.

A lot of points can be mentioned in here which can shed light on the issue you are referring to. For example, how many days or weeks you were at your previous position because temporary placements could shift a lot during days and nights but rather fixed spots tend to remain almost untouched. Have you done anything very quick in recent days, like building too many links for your website? You see, even the trends in building links can play big roles over the net many times.

A sample website which has been stable online and then suddenly in one way hundreds of new links are added to its pages, well, the trend seems a bit unnatural, wouldn't you agree? Maybe it is a temporary issue, especially if your site is rather new over the net, has not been online for at least a year. One more thing which sometimes is the case is about your older links, try to make sure they still are in place, sometimes when some links are gone, so will be the ranking online.