Exactly the same thing happened to me yesterday as well.
1st Transaction $103 ITunes
2nd Transaction $206 ITunes denied by bank.
I asked about it and they claimed they have people monitoring transactions and they notice unusual activity which is out of the normal pattern of purchases. I called BS, but they played dumb.
It appears that we did not have anything charged to the card. They need the 3 digit security code on the back if they are going to charge something online (ideally) and they need the 4 digit code to use it as a debit card. We tend to use the card to try to pay cash for gas and restaurant expenses, so now it will be 7 to 10 days before we get another card. Bummer.