This may sound a bit nuts but I honestly think Trump is doing this because immigration, foreign policy, and commander in chief are the three areas where he has nearly dictatorial powers, so he simply chose to turn those cranks because those were the ones available.

If you look at how he is turning the cranks it doesn't make sense, virtually no modern economic thought supports this kind of tariff and customs war with friendly Canada. It seems like a child playing at the wheel of a car with no idea how to drive but by God you will hear him honking that horn.