Charging $18,000 and paying it off "a few days later" and not when the statement closed, might have triggered a manual review. I'm not accusing you of any wrongdoing, nor is Amex, but it can take weeks for someone to check the charges. They will be looking for things like cash-equivalents, gift cards, etc.

Have you ever had a Gold Skymiles card before? Amex is very specific that sign-up bonuses are once per lifetime.

Oh and welcome to Flyertalk!!