Why We Ask You to Re-enter Your Card Number
If you request delivery to a new address (one that you haven't used with us before with this credit or debit card) or want to send an e-mail gift certificate to a new e-mail address, we'll ask you to reconfirm the credit or debit card details.
That way, should some third party guess your password and try to order items using your account, their attempt will fail. We hope you understand that this is a valuable security precaution designed to protect our customers.
Courtesy of amazon.co.uk.