What causes the Amazon price drop in video games?

On Amazon, sometimes prices of video games like LA Noire did last week and this week Alice: Madness Returns will drop by about 20 bucks while on other sites like Walmart and Gamestop they are the same price. Within the next coming week though, the game will shoot back up to close its original price. These are new games mind you, so what causes this price drop on Amazon and how can I plan for it?