These bonds generally have a fixed term of up to 30 years, under what scenario would the US have to repay its bonds all at once, before they're due?
It wouldn't. The general threat is that China would sell these treasuries on the market and flood the supply. That would make it much harder for the US to sell new treasuries (which it does every quarter) and so the discount it would have to give would be higher (the bond yield would go up) and that would mean more expensive debt repayments in the future.