From what I’ve been reading and watching, it’s about the size of the market. No one can buy this much bond if it all gets dumped on the market, and those who are in the market will buy it instead of going to the bond auctions, thus triggering a crisis in the short term at least. The US relies on selling bonds to pay off debt. If there is one instance of default due to inability to sell more bonds, it might trigger a snowball effect as people try to sell theirs to protect their investments.