I am sometimes asked why some websites drop for certain keywords and the answer is one of 3 things. You need to remember your site hasn’t dropped, but other sites have moved up. Your site is in competition with every other site out there, so websites will go up and they will also go down. The only way your site will stay at the top is if it is the best site out there, or you are paying more than everyone else to market it.
Some of the reasons why a website has dropped in rankings are:
1) It could be some of the links pointing to your site which previously had been providing a significant amount of PageRank could have been removed or deleted, or the PageRank of the originating site has dropped possible for similar reasons.
2) Search engines are always changing the methods they use to rank websites so they can improve the general quality of their search results. So there could have been a recent algorithm change. Google is constantly changing its algorithm.
3) Several new web pages that also target your keywords could have been recently created and indexed making that keyword/phrase more competitive.