The foreign bond holder sits in a very different position than a domestic one.
Any holder of an existing bond will see that instrument lose value if interest rates spike, opening the door to them selling that bond reducing the prices, and making rates spike even higher.
But the foreign bond holder also generally will convert the proceeds back to their currency, which they acquire by selling dollars, which drives down their price as well.
This entangles current holders of the dollar, because now they're assets are losing price too.
Do that enough and all US paper assets denominated in dollars, such as stocks and commodities, start losing value, which opens the door to a broad selloff of all those assets and massive cascading crashes across the board if there is a panic.
That can then snowball to multiple consequences which can reverberate into the real economy, real fast with very negative and possibly cataclysmic effects much like the fallout from the '29 crash, except on a huge multiple v. back then.
We are playing a very dangerous game with all this sudden explosion of debt.