Keywords can fluctuate on SERP (Search Engine Results Pages), as search engines now continuously updates their algorithm. Stabilization of Keywords is not guaranteed unless you try some paid services (Google Adwords, Facebook Adwords or Bing Adwords)
However, if you want your important Keywords to be resulted in SERP listing, then you must build content accordingly on your Webpage, and approach quality link building efforts so as to rank higher in Search Engines.
It takes a while for a site to settle into its home.
If you set up the site and started building links and getting social signals to it then google will pay attention, but if you stop or slow down the momentum they will realize that and think it was just a hot site but is no longer as valid as it was.
It will continue to fluctuate for a while until they are comfortable with where it ranks among the other sites.
If you are in a highly competitive niche and there are a lot of others working and ranking for the same thing it will affect where you stand as well. You will need to play with the keywords.