During busy ordering times (especially on the first release of a new item) product moves quickly, and when you place an item in your cart it does not reserve the item for you. There could be times when the item is sold out by the time you reach the checkout process. When that happens the item is removed from your cart to prevent overselling.
We're incredibly sorry for this inconvenience. You can get notified when the item is available again by entering your email address within the "Get Notified" area on the product page.