Of course, a single short seller cannot “make” the price of an asset (unless he is George Soros, whose famous bet against the British pound in 1992 made him a billionaire and forced Britain out of the European exchange-rate mechanism). But if a bunch of speculators decide (rightly or wrongly) that a government’s debt is overpriced, they can force down its price, thereby forcing up its yield (the interest rate that the government must pay).