David, The reason it happened is because Google is making considerable changes to their Algorithm. One of the main reasons for the change in Algorithm is to weed out the spammers who try every trick in the book to get sites top for certain keywords. It makes sense then that during a major update like this that some sites will drop and some will rise constantly whilst the update takes effect. What falls out the end of the update is hopefully more precise results, so if you built your site naturally with good content you have a good chance of bouncing back. If you don't bounce back you need to revisit the basic
SEO techniques which Google themselves publicise on their own site and then change things around. As I have said for a few weeks though you have to let the update level out, as hard as it is you can see now why....your results are showing again and with a bit of luck they will stick!