Why did he threaten 100% tariffs?
Because Canada has been in trade talks with China and may potentially lower its tariffs on China which gives them a back door into the US. There are some specifics and it's all conditional. It depends on the kinds of deals it settles on.
I do feel as why demands reason and I am not sure if you can reason with the unreasonable which is what the Canadian speech was about in Davos and then POTUS threatened 100% tariffs again.
Kind of proved the point of America being an un-reliable partner which is what I inferred from Canadian PM's speech & his call for middle economies to connect with each other and strengthen together to have more leverage overall.
Because after the US threatened to destroy our economy and/or annex us by force and/or cancel nu-NAFTA and/or impose tariffs on us regardless, we realized that Americans don't actually want us as friends so we started diversifying our trade partnerships and negotiated a mutual tariff relaxation deal with China.
The previous Canada-US relationship is gone. Months ago I wrote on HN that purely by virtue of having to weather this storm, the nature of Canada-US relations will be irrevocably and fundamentally altered. Even if Trump and his cronies were jailed tomorrow, it's too late. The rest of the world understands that Trump is just a symptom of the disease affecting America and it's going to get worse, not better.
Delusion? Dementia? Being surrounded by yes-men?
He did not like the Canadian prime minister's speech about "great powers" weaponizing economic integration, so he decided to prove him right.