As I understand it, the story is as follows. Note this is mainly opinion, backed by some slim facts.
Canada started buying US treasuries months ago, ostensibly for the purpose of financial security.
Shortly after Mark Carney became prime minister, he travelled to Great Britain and France. These were not photo-ops. As someone with a great deal of background in international finance (he was governor of the Bank of Canada and the Bank of England), he had connections. Discussions were had.
When Trump announced his idiotic tariffs, the stock market plunged of course. When stocks plunge, normally people retreat to the safety of US treasuries, and the price of them goes up, making interest rates go down. This would have been OK with the US administration, as it would have led to cheaper interest rates, and less money going to pay the debt.
HOWEVER. Canada, UK and France sold off US treasuries from their stockpile in the face of the stock market drop, which is totally contrary to what was expected. This led to a decrease in the price of treasuries, and an INCREASE in interest rates. This was entirely unexpected, and led people with financial knowledge to completely freak out. This then led directly to Trump’s reversal of course before 24 hours was out. There was the danger of a complete and total meltdown, as has happened in the past.
Canada and friends played the game - showing without announcement or fanfare that there are tools at the world’s disposal to crush the US economy whenever we chose.
Interestingly, just a day or so ago, Japan (largest US bondholder) announced that they do not intend to sell their massive US bond holding. The message was pretty clear - they did not do the selling (this time), and the unspoken part was “we will not sell … YET”.
The Trumpers are picking a fight with the entire world at this point. They’re not going to win this.