It began with a rush in mid-March, when a pair of U.S. corporate giants, Exxon Mobil and Verizon Communications, braved the financial turmoil created by the coronavirus pandemic and sold a combined $12 billion of bonds in a single day. Others quickly followed, emboldened by the unprecedented support provided by the Federal Reserve, and before long, deals were being rushed out at a clip never before seen in the history of U.S. bond markets.