For all the nail-biting about the economy over the past year, investors had it easy: a 5% yield on short-term U.S. debt, one of the safest investments in the world. Now, the times they are a-changin’—again.

Borrowing rates have already fallen in anticipation of a deeper cut in rates. The average 30-year mortgage rate dropped to 6.2% last week to the lowest level in 18 months and down from a peak of nearly 7.8%, according to the mortgage giant Freddie Mac.