Jason Zweig explains how investors have swarmed high-yield bonds this year, pouring more than $20 billion into the asset class. "The inflow of funds has been phenomenal," said Daniel Fuss, manager of Loomis Sayles Bond Fund. "I have not seen a rally like this, ever, in the high-yield market." However, the rally might be about to end. "As they so often do, investors are piling in even as the party may be starting to wind down," Zweig writes.