The most likely reason is that your order contains a pre-order title or a title from a recent LiveSigning.
As stated on the product page for pre-order products, orders containing pre-orders won't ship until all pre-orders in the order have released.
The other likely reason is that your order contains a title that was recently released or had a recent LiveSigning. Recently released titles that did not have a LiveSigning typically ship during the release week or the following week, though occasionally a delay will occur. Orders for recent LiveSigning titles typically ship 1-3 weeks after the LiveSigning to allow the author time to sign, but occasionally ship later if the author takes more time than usual to finish signing. You will receive an email with tracking details when your order ships.