I see these threads all the time, a webmaster complaining their rankings suddenly dropped and for no reason.
Well, Google's John Mueller responded to one of these, potentially with good news but also Google-like advice.
In a Google Webmaster Help thread John said:
Fluctuations like this can be normal - the algorithms don't stand still and use many signals to review & determine the relevance of pages & sites. Looking at it just now, it seems that it may just have come back again :). That said, working to improve the overall quality of a website is always a good thing. If this is currently one of your busier times, then it might make sense to do that a bit afterwards -- but I'd definitely think about what you could do to get feedback from users (customers or not) already.
I checked the site's rankings and it was not back to number 9 for me. But maybe John saw it back at 9. Either way, the advice is typical advice from Google but important.
In short, Google's algorithms don't stand still, neither should you.
You should keep working on your site, making it better for users and hopefully Google will appreciate those efforts as the algorithms update.
Forum discussion at Google Webmaster Help.
