I called the credit card company and they'd couldn't see an attempt from Iberia.

The wording on the pop-up was "We have detected a possible improper use of your payment card. For your security we have not allowed the completion of the sale online with this payment card". So I think it was Iberia blocking it.

(I had used (and saved) the same payment card successfully a couple of months earlier for multiple cash bookings but was at home at the time).

I am not sure but I suspect that BA allowed Iberia to block the seat for about 2 hours so that I could complete the booking using a different card / calling them / going to their offices.