I'm not sure how much I can copy and paste from the NY Times site without running into copyright / fair use issues. Here's the article from the second and third paragraph:

"In recent months, American Express has gone far beyond simply checking your credit score and making sure you pay on time. The company has been looking at home prices in your area, the type of mortgage lender you’re using and whether small-business card customers work in an industry under siege. It has also been looking at how you spend your money, searching for patterns or similarities to other customers who have trouble paying their bills.

"In some instances, if it didn’t like what it was seeing, the company has cut customer credit lines. It laid out this logic in letters that infuriated many of the cardholders who received them. 'Other customers who have used their card at establishments where you recently shopped,' one of those letters said, 'have a poor repayment history with American Express.' "

Cheers,
Pogopossum