Bill Trbovich, a spokesman for the International Association of Machinists and Aerospace Workers, said when there's no more room for carry-on luggage inside a plane, handlers are forced to take the rest outside in a rush to be stowed elsewhere so the flight isn't delayed.
But the stairs up to the plane are so steep, it would be dangerous to hurry down, Trbovich said.
Meanwhile, "management is pushing them to get the planes out on time," he said.
The video, taken by a passenger on board a plane at Toronto's Pearson Airport, shows one worker dropping baggage about six metres from the stairs to a luggage bin on the ground while another baggage handler works below.
Air Canada, which has dealt with backlash since the video was posted on YouTube on April 18, said the two employees have been suspended and will be fired as a result of the incident.
They have been told "their employment will be terminated pending the outcome of our investigation," spokeswoman Angela Mah said in an email.
The airline has apologized for what it calls the "totally unacceptable mishandling of our passengers' baggage captured on video."
The video is titled "How Air Canada Handles Your Baggage" and has prompted some to vow never to fly with Air Canada again.
Some, however, have come to the airline's defence, saying it shouldn't be judged by one incident.
Trbovich said it's not yet known whether the union will file a grievance.
But he stressed the airline could help prevent such incidents by enforcing its own carry-on restrictions.
Employees are also under increased scrutiny now that everyone can shoot photos and video on their phone, and it's not enough to just do your job safely and to the best of your abilities, he said.
"You're constantly looking over your shoulder," he said.
"You go about your business trying to do your job and you've got to be cognizant of where you are to make sure you don't hurt yourself or fall down or something, and to worry about whether somebody's taking your picture through an airplane window."
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