If you don't update your site does Google drop you out of the top 100 listings ?

This happened to me as well. I was on the first page for a particular keyword and for a while I didn't write anything about that keyword, but my blog was kept updated nonetheless. However, since my blog didn't give out anything new about that keyword, I got pushed onto the fifth page after two months! I still don't understand the logic but I realise that it's important to keep the blog updated in particular keywords as well.