I know this is Trump, so the answer is ¯\_(ツ)_/¯. But over the past two weeks his wrath has been aimed at Denmark, Colombia, and now Canada and Mexico—all of them among our staunchest allies. You could hardly pick a group of better friends to the US than these.
On a concrete basis, Denmark has agreed to increase its defense spending to 2%, meeting a Trump demand. Colombia has been accepting US deportation flights for years. Mexico has agreed to reinstate Trump's beloved Remain in Mexico policy. And Canada has been cooperating on the fentanyl front even though nobody thinks there's very much it can do. The United States simply doesn't have any big gripes against any of these countries.
Shouldn't Trump try to pick on at least a few countries that we don't actually like very much? Why the focus on longtime friends and allies?
Our enemies have already bribed him, so he's leaving them alone. Our friends haven't bribed him, so he will pick on them until they pay up.
International relations under Trump is a simple protection racket. Pay up and you get left alone. Fail to pay and you get roughed up.