The premier of Manitoba advanced the idea that he's doing that on purpose to enrich his billionaire friends. Have American consumers pay more for their products to cover the tariff, then a few months later the tariff is deemed illegal, then the big corporations get a refund which they do not pass back to the consumer.
IMO I think it's a combination of people in his inner circle feeding him bad ideas, manipulating him into attacking Canada's economy. Some in hopes that a collapse of Canada's economy would allow them to turn it into a subservient resource colony, others because pushing away America's closest ally serves Russia's interest.