Bangladesh, Sept. 17 -- The most important number in global finance this week may not be an inflation reading, an oil-price forecast or even the Federal Reserve's next interest-rate decision. It is the yield on the 10-year US Treasury note.

On September 15, the benchmark yield briefly reached 5.041%, its highest level since 2007, before retreating toward 5%. That move is significant not simply because of the number itself, but because the Treasury market sits at the center of the global financial system. When the cost of borrowing for the US government rises sharply, the consequences can travel through mortgages, corporate loans, currencies, equity markets and government finances around the world.

What makes the current bond sell-off p...