Rankings change, it happens. Sometimes that's because the algorithms themselves change, but more often it's because the data being fed into them changes. Links to your site may have dropped in value, links to other sites may have sprung up in high-rent districts. Other sites may have expanded their content or improved their search-friendliness. A page that was once at the top spot might not stay there. SEO doesn't stop, it's always a case of "running to stand still", if you don't keep trying to push forwards, the chances are you'll slip backwards.
Google's job is to provide the search results that give users the pages they're looking for - their job is not to make life easy for webmasters!