Boycott US

Supply and demand answers the question. When there is a large supply with no change in demand, prices drop. When there is a short supply with no change in demand, prices rise.

There's not going to be any reason for a sudden rise in demand for US Treasury Bills, likely, at the same moment that Japan dumps its 4.3% of the US Treasury bills on the open market. And a sudden sale of 4.3% is going to cause a precipitous drop in the value of the bills. (This is going to be even worse if China takes that opportunity to dump its own 2.9%.)

And then comes the free-fall.

Because if T-bills drop suddenly in value, other holders (like the UK, with 3.1%) will sell off quickly to avoid taking a bath. Which will make the value drop more quickly causing other people to sell off etc. etc. etc. To make up for this, the US is going to have to do some serious work. With a free fall in price, the US Treasury is going to have to boost interest rates a lot to make up for it. This makes it more expensive for the USA to borrow money to finance its government operations. Interest bills on the national debt go up. (So does the deficit budget, probably, or the US starts killing its citizens instead the Trump way).

Oh, and that's just governmental cost. Private borrowing starts getting more expensive too. Loans have higher interest rates, which will negatively impact all businesses big and small. Which will eventually negatively impact everybody.

But ... this will hurt Japan (and China, say) as well, right? I mean selling off the T-bills will raise the Yen/Yuan and hurt the Japanese/Chinese economy, right?

Traditionally, yes, that would be true. But, see, the USA's idiot of a president now has tariffs greater than 100% on China and is threatening 35% even on Japan. The damage on exporting is already done!. Because a bumped up currency hurts exports, but if exports have already been virtually eliminated, there's nothing that a bit of a currency rise is going to really do anymore. Trump's "beautiful" tariffs have made the sudden sell-off of T-bills more likely and given foreign debt owners (Japan, the UK, and China in decreasing order of proportion) a more powerful weapon as a result.

Combine this with the retaliatory tariffs and the US economy is being ass-fucked sideways with a rusting spiked dildo if Japan (and probably China) suddenly drop their T-bill load.