The U.S. Treasury prefers its debt sales to be humdrum affairs. Lately, they are sparking fireworks in markets.  

Scrutiny of Treasury auctions—whereby the government funds operations by selling the world’s safest bonds to big banks and dealers—has grown alongside their size. For years, many in Washington and on Wall Street assumed that investors would buy any number of bonds the government issued, no matter the fiscal outlook. Testing that assumption: the sale of $20.8 trillion of new Treasurys in the first 11 months of the year—set to surpass 2020’s record of just under $21 trillion.