[hboon]

It's due to the nature of the market place: hits-based due to the charting nature of iTunes. So you typically get a huge boost in exposure when you launch (it's in the "new" chart as well as due to the PR that is drummed up externally) and as long as the resulting sales is enough to give a boost to it's ranking (in the "top X in category" chart, "top X in app store" charts, as well as being featured, which is really another chart), it keeps climbing. If downloads is sustainable at a certain level, the app settles at a particular ranking and earnings. Otherwise it fades and becomes forgotten. Some type of apps — games, novelty apps — are more susceptible to this.

Think of it like music or books. Apple is selling apps (to buyers) like it sells music.