For some time now, DL has required non-Medallions purchasing a ticket within a few days of travel to show the CC and an ID at checkin, apparently as a fraud-prevention measure. Perhaps this policy now applies to NW ticket purchases.
From
DL.com:
Credit Card Presentation
To safeguard against credit card fraud, the purchaser may have to show us the credit card along with a valid photo ID. The time varies based on the billing address of the credit card or the country of travel. If the purchaser is not traveling, they can show us their credit card and ID at an airport ticket counter or another ticket office location, whichever is most convenient.