I have noticed while tracking the rankings of various keywords lately there seems to be a lot of movement that can happen. For example a couple of weeks ago words that I was ranking for on Page 1 quickly dropped to page 3 and have now moved up to Page 2.
There seems to be quite a settling in period particularly for keywords that you have just started to rank for. You may find that initially you will rank very well for them, then suddenly drop.
This from what I can gather appears to be a normal process, its similar to how new sites can initially appear very well in the Google rankings, then they drop into obscurity.