I had the same thing happen on 2/1 when I was purchasing several one way tickets on alaskaair.com. My Alaska Air VISA was declined after the second purchase. I contacted the card issuer, Bank of America, and was told the database was down for routine maintenance, so I couldn't find out why the charge was denied. But strangely, I was able to use the same VISA at Amazon.com not a half hour later with no problems. I did contact both Alaska Air and BofA the next day, but but neither had any explanation for the event.

In this case, I was able to hold the tickets for another 24 hrs. at the same fare, but I can see how this might cause real problems should it happen again.