Google is not going to keep you at number 1 or even the 1st page if you do not keep up with all the updates to the algorithms and in all fairness they do tend to rotate sites somewhat.
Have you removed or made changes to the index or structure of the site?
We noticed a drop and then a surge in the past couple of months. Something changed and then it was back to normal. It's not easy to be number one but I think in your case it will require some digging to see just what happened.
Possibly a competitor or new competitors have improved
SEO?
Might check here for all kinds of ideas regarding Algorithms.
Google Penalty Checklist: Rank Drop Troubleshooting Guide