In the weeks and months that are leading up to the release of a big game (like Grand Theft Auto 4) a lot of people are buying the system (in part) in anticipation for the release of this game; when the game is released these sales are transformed into people buying a system because of the game. Over time the number of people who are buying a system to play a particular game drops at a fairly steady rate, and to maintain the sales rate new games have to be released at a steady pace to replace these 'lost' sales with new 'anticipation based' sales.
Hypothetically speaking, if you released several really large games and had little for people to look forward to in the medium or long term you could see (somewhat rapidly) declining sales for the system over time.
