Why your Google rankings drop and what you can do about it

[Ricechen]

1. You lose the fresh content bonus.
2. Your internal link structure changes.
3. The link structure on sites that are linking to you changes.
4. Duplicate [i.e., stolen] content.
5. Your site becomes stale.

[mokmok69]

Actually optimization is a continues process that's why you don't have to stop optimizing your site,...any ways look new guidelines or updates on Google.com

[doomcashdown]

You can also include "page jacking". If you are unlucky that your website got hi-jacked by hackers, the rankings can go to their website instead of yours, thus, rendering all of your efforts to rank your site useless until you solved the case.