FWIW, I suspect it is an immigration rule. On AA, AC, AS, BA, and UA flights from DUB, YUL, YVR, and YYC departures to the U.S., I'm asked to show my passport before boarding. The only time recently I don't recall this occurring out of a Canadian airport was at YTZ for an EWR flight. But Billy Bishop didn't have a customs pre-clearance.