This is definitely a question for Amex customer service, since only they are going to know the details of the situation.
Anything we tell you would be modestly informed speculation at best.
Did Amex even send you an email about the large purchase? Just last month, although it says I have NPSL, I got a call card issuer message for a $824 charge...my statement had just cut and although I had paid 1000 off my 3690 due I figured let me try and see if they would put that through...but when i called them the guy said let me see what I can do, he went on to say I see that you had an unusual spending pattern, i told him yes had some car troubles this month on both cars, he goes ok well i will approve the charge, just go back to the merchant and run the charge again..then asked when will i make a payment, told him the thursday i will pay 1700 and by the following monday pay the balance, way before the due date too and he said fine. All this to say that maybe your account got reviewed and they decided to just let it go....be happy about it eh.
So you have been stringing the merchant along for three weeks? They probably got tired of it and submitted it as a force post without a preauthorization. Sometimes it works, sometimes it doesn't. And it can seem to work and backfire later.
You aren't supposed to do it, but can if you have the proper permissions on their terminal.