Professional SEO experts refer to this as the Google dance. It's a difficult thing to explain to clients, especially if they tend to believe...
- We deserve to be on page one, so any work you've done to get us there is incidental
- We've fallen off page one! What did you do?
Of course, the Google dance is totally out of your control and mine. It happens when Google is rebuilding its rankings. Sites tend to jump up and down the listings during this time. It's nothing to worry about, and your only option is to wait for it to stop and see where you end up.
If the fall is permanent, you have some work to do. If you've risen in the rankings, you can relax for a while - or start work on getting even higher next time.