Your keyword is not static because keyword that you targeting are target by many webmasters. And all work hard to rank their website on top. So you also have to do hard work every day and check your competitor backlinks that from which sites they are getting links and what they are doing that boosting their ranking.
It is part of game you can't never stable your keywords on a particular ranking position your keywords obviously change their ranking sometimes up sometimes down because it depends on your work you do for good ranking positions in SERP.