I have a hunch that Amex tracks your activity on their website when you are logged in.
I had credit cards with Amex for 6-7 years but never used them. I then started poking around the Amex site looking at the Gold charge cards. I then got inundated with sign up bonuses for the Gold card, which I did not accept for various reasons.
Fast forward a year later, I started looking at the Plat benefits on the website. I received two generous MR offers to accept the Plat card within as many months. YMMV.