Some of the world’s top hedge-fund managers are worried. They say central bank bond buying programs that are pumping trillions of dollars into global markets will end badly.

The European Central Bank said Thursday it would extend its asset purchase program to the end of next year, buying bonds at a reduced rate. Prominent managers have told The Wall Street Journal in recent interviews of their doubts about the endgame for quantitative easing around the world.

“There’s...