1. You did something wrong – Have you made any recent changes to the website? Where have you recently been building links?
2. Your competitors did something half decent – Have they made any significant on page changes, perhaps a website refresh? Maybe they have increased their link buyin… building budget?
3. The search engine changed how it judges webpage’s – From time to time search engines like to improve their algorithm’s to give more relevant results. If it is not the 2 points above then an algorithm update is quite likely.

And how to fix them (this is if you are 110% sure it is a certain reason):

1. You did something wrong – Did you over optimise the page? If you have made changes to the site in the last 4/8 weeks its probably a good idea to go back to the original. Not made any onsite changes? Get in contact with all websites that you have used to gain links over the last month/3 months/year and kindly ask them to remove the link pointing to your site (however “quality it may seem”). Monitor results, see increase, learn lesson.
2. Your competitors did something half decent – Review the competitors on page content, steal it, improve it and add it to your site. If they haven’t changed their content then review their link profile in Majestic SEO or Open Site Explorer. Have they seen a spike in quantity? If not manually review for standout “quality” links. Depending on the result either get more links or get more quality links 😉
3. The search engine changed how it judges webpages – Has there been a recent update? Are people crying to high heaven in Digital Point and Webmaster World? Have you seen similar results with other clients? Check out the main SEO blogs or Matt Cutts/inside search to try and find out a reason ASAP. Search for others who have had similar issues, review their sites and look for parallels between your site and theirs. Maybe tweet Mr Cutts to see if he can shed some light on it… Make notes and make changes. However if there doesn’t appear to be an algorithm change make up your own theory and add it to your blog. If you have a decent readership people will agree (regardless of being effected or not) and this in turn will convince the search engine to write a press release on their latest update.

This hasn’t covered every possible reason for a loss in rankings but it will help explain 90%. Now get to work and fix that problem! 🙂