There are a few factors that come into account and that can influence your current website’s Search Engine ranking:
1. When your website’s content and/or structure changes significantly
It can be expected to see a drop in some of your website’s rankings, just because search engines update their registries and re-evalutate the site. Consistency is one of the most important factors of SEO, and if there is a significant change on your website, Search Engines may consider the site as less valuable for a while. I am not talking here about updating your website while remaining consistent with your targeted keywords – this is a very good practice that ensures that your website stays “alive” and relevant in the eyes of major Search Engines.
2. Search Engine algorithms change
When the algorithms change, so do the search results, and ranking can sometimes vary very significantly. That is one of the reasons why SEO is (and must be) considered an ongoing work. Both on-page and off-page techniques must be consistently applied and the website must continue to grow in popularity, traffic, content, etc.
3. If you’re not working on your site’s SEO, it’s a sure bet your competitors are
It is very likely that your competitors are working on their website’s SEO and improving their ranking – leaving you behind. Again, SEO (like most advertisement or promotion technique) is something that should be worked on continuously – even if only on a low flame, just to maintain your status.