My understanding is that Air Canada offering a USA to USA trip is cabotage. And a US airline offering a Canada to Canada trip is also cabotage. So if the OP is on one ticket on US originating and ending in Canada, then it's cabotage.

If, let's say, the carrier was Air Canada and it was something like YOW-PHX-YVR, then it wouldn't be cabotage, but AC doesn't offer domestic flights with a US connection because it isn't in their, nor their passengers' best interests (mileage runs aside ).

If I'm wrong on this, please educate me.