The official statement from the White House calls out increased fentanyl production in Canada as the reason - but doesn't say that it's the majority of imports, you're right. The twitter summary says "There is growing production of fentanyl in Canada, and enough fentanyl was seized at the northern border last fiscal year to kill 9.8 million Americans.", though, and that seems like a lot.
https://www.whitehouse.gov/fact-sheets/2025/02/fact-sheet-pr...