Hi Georgia: A similar thing happened to me just from buying a 5k bond on the HHonors Amex with a 13.5K limit. It was the only thing on that card and they called right away the next morning. They required me to send in my last years' taxes and recently bank statements. In the meantime, my Amex cards were frozen.
It took about a week and they called and said everything was fine and sorry for the inconvenience.
The woman admitted this was triggered by the bond purchase. Never happened before and hope not again as I still have 30K to go this year.