Agreed with
@Albort that FR is a bit overblown. When it happens, people freak out and you'll read about it on the message boards. Some triggers seem to be cycling your credit lines, too high credit line on a card, high utilization on a card, high spend relative to reported income, checking the spending power on a charge card for a big purchase, paying your cards from several different bank accounts, non-payment, etc. It's basically engaging in what any sensible bank might consider risky behavior. Amex will ask you to produce documents showing income, e.g., bank statements, tax returns, etc., to show you have the ability to pay and haven't exaggerated your income, and they'll either close cards or reduce CLs on cards if they still think you're a risk.