“I would characterize it as concerning,” said Jonathan Hill, U.S. rates strategist at BMO Capital Markets in New York. “Repo should not be swinging multiple percentage points a day.”

The market for these instruments serves as a vital source of short-term funding for banks and hedge funds, which borrow billions each day at very low rates that fluctuate in part with the amount of money that is available for lending.

Usually these changes go unnoticed. But at times, the repo market has served as an indicator of growing stress in the financial system. During the financial crisis, unexpected changes in short-term borrowing costs provided some of the earliest indication of significant problems on Wall Street, as investors grew leery of lending to prominent financial institutions.

The move on Monday was partly attributed to a few idiosyncratic factors. It was an Internal Revenue Service deadline for quarterly corporate tax payments, and a settlement day for recent auctions of United States Treasury securities. A holiday in Japan also reduced the supply of cash to the market.

Still, the market should have been able to adjust to those factors without a surge in rates.

“It should be a very predictable day for cash being taken out of the system,” said Subadra Rajappa, head of U.S. rates strategy at Société Générale in New York. “But the magnitude of the spike is what is quite surprising.”