NEW YORK — A warning from Standard & Poor’s that mounting debts put the U.S. government’s credit rating at risk blindsided markets last Monday. The Dow Jones industrial average lost more than 240 points in the morning before recovering. It was the worst one-day drop for stocks since fears over a nuclear meltdown in Japan sent investors into hiding on March 16. The response made sense. A downgrade of U.S. debt, after all, could turn into an economic calamity. Here’s the surprising part: After a quick dip, prices for U.S. government debt began rising.

Economists and bond traders offer varying explanations for the Treasury market’s curious reaction, but there’s a common thread: S&P’s warning shot could actually wind up making bonds more attractive.