Often people laugh around discomfort and slavery is a discomfortable subject. So my guess is something in the way you said blacks were packed as animals, like animals, or the phrasing set off the class into nervous laughter. It could have to do with comparing them to animals as they were treated like animals so it's a little too close to home and uncomfortable; again something that causes nervous laughing.
There could also be something you said that was nuanced in a way that it made it a joke but the fact that you don't understand it would make it hard for you to identify and relate it to us so that we can understand.
Probably because animals back than were treated much better with more respect and care.
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