By any chance, is you or someone else that has a card attached to the account a non-resident? I've seen an uptick lately in KYC procedures for non-residents, from Amex but also from legacy banks. One of the outcomes of failing that procedure (which may be tricky from time to time) is that it can result in your account's suspension without further explanation. I'm not 100% sure how American Express is regulated, but if it is like any other bank, they are not supposed to share why they are shutting down an account.