Sadly a lot of fraudulent transactions involves WoW - foreign crime syndicates creating WoW accounts using stolen credit cards and reselling these accounts to Chinese farming companies, etc. So thats why many banks and credit cards are very quick to stop any transactions, especially if you haven't done any before - most credit card companies nowadays "track" customer behaviour, so for example if every day you buy a starbucks coffee in Portland, but suddenly one day you buy $200 worth of books at a New York City bookstore it'll get flagged.

So yeah, your bank/cc decided something must be suspicious and didn't let the charge go through, could be something as simple as the address/phone/zip not matching or just routine bank flagging. Its better as someone said to buy retail WoW boxes at stores, its cheaper ($25-39 for the entire battlechest) and you have 0 delays when upgrading.