In terms of falling in recruiting rankings, it generally happens for a couple of reasons. One is that other recruits may have committed to other schools that were near us in the standings and therefore, pushed them ahead of us. Second, our decommits have also committed to other programs, which probably has also hurt us a bit. Third, Rivals (and Scout for that matter) periodically update their ratings for recruits and depending upon whether players go up or down in Rivals' eyes, the ranking could be affected that way.
Having said all that, I don't put as much stock into recruiting rankings as others. I'm more interested in how they ACTUALLY perform on the field as opposed to how someone else thinks they MIGHT perform on the field. Still, it's an interesting talking point if nothing else.