I see it as something similar to some of the motives driving Putin with Ukraine. All the rhetoric about "NATO at our doorstep" is just a smokescreen for what the real fear is -- he cannot countenance an alternative Russian/Ukrainian speaking polity, culturally-partially-contiguous with Russia to exist on his doorstep if it is a liberal democracy, outside of his sphere of (corrupt) control, and not subject to his kleptocracy. Because it would be an internal threat. The cost of grinding Ukraine into the ground is worth it to him if it means maintaining strict control at home.

What Trump is doing is like a kid's colouring book version of the same thing. It's crude jingoism to shore up his own base with bullshit about Canada and whatever, to build legitimacy based on jingoistic nationalism, and to try to undermine and destroy a liberal / centrist gov't on his doorstep. And, consistent with his "drill baby drill" mantra, it's also an attempt to stop climate change initiatives, to free US capital in Alberta from Canadian regulation, and to maintain/extend American control over our resource sector.