If the site isn't updated very often, it will get indexed less often as well. This gives other sites a chance to catch up.
Also, if the content on those pages is reproduced on a lot of other pages throughout the internet, Google will think less of the page and it's rating will go down.
People may have put links to your site on there site so your Google Page Rank will have gone up. However, the links may now have been removed, so Google thinks your site isn't important any more.
My guess would be your content is being used elsewhere or you are using other peoples content. Try searching on Google for:
related: www.mydomain.com
Obviously replace www.mydomain.com with whatever your domain is!
Keep updating your site regularly and use unique content. Also, try to get as many good links to your site as possible. Make sure the links are from sites that are about the same topics as yours is, with your keywords in the link and are not from sites that reproduce other people's content.
Remember, Search Engine Optimisation cannot be guaranteed, the Internet is not predictable and it takes time to get a website established.