I'm pretty sure it's not a straight credit risk denial.
Instead, I'm betting it's an act to avoid / discourage a consumer that their model suggests travels a great deal relative to the amount said consumer spends on travel.
Roughly translated, my bet is that they think you're gaming the sign-up rewards and that their automated risk model has identified that and blocked you for it.
I'm not sure whether it's relevant, but I read an article on the topic that alludes to Amex using new / improved data and analytics to crack down on this sort of thing: https://www.smartertravel.com/2017/03/10/travel-hackers-beware-american-express-eyes/
