There are probably too many factors to pin point why your SERP would fall or even rise on that fact. Just contiune daily to improve your content, increase inbound links and it will take its natural course.
Maybe your site lost some links. Maybe your links have been devalued. Maybe other sites did some better SEO. Maybe your content is not good for that particular keyword you are after. Maybe your content is not being regularly updated (remember google loves this). Keep in mind that SERPs are constantly changing. Especially for the competitive keywords. Also the age of your site plays a big role to this. Google favors the older ones as being more reliable. Keeping a good SERP is an ongoing job. It's very easy to go from page #1 to #10 for a competitve keyword if you stop working. So, for now I would suggest you spot your site's position and take small steps trying to improve it. Make sure you monitor the results of each move you make. That way you will find all answers you need to have about SEO and SERPs and understand which things are beneficial to you or not.
this is maybe the following reason you might be experiencing: - google updates SERPs gradually. sometimes your site is nowhere to be seen or moved then after a while, there, it goes back. - you were over-ranked by your competitors (maybe because you aren't updating your site regularly.)
By this I mean that some certain kind of links have lost their value in terms of boosting your SERPs. For example if links come from link exchange they have no value when it comes to SERPs (they used to some time before). The same applies to paid links. Generally sitewide links have less SERP determining value than single page links, as sitewides are most often a paid form of advertisement rather than natural links. I don't know if I managed to explain this to you. Feel free to ask anything you want to know on this.
If you are exchanging links, go to http://www.iwebtool.com/visual_pagerank adn check that all the links you have on your outgoing links page all have pr. If they do not, chances are that the site you are linking to is considered in a bad neighbourhood and everyone exchanging links with that site are penalised. Check that the pages are in the google index, and if they are not, they have been penalised. Get rid of them immediately. You will find your rankings will go up again. Happened to me, and now I check every month or so.
http://forums.digitalpoint.com/showthread.php?t=530832 the same to me. what is important :how long did it cost to come back? long time will kill a seo company.