It’s very simple. You could fall, pitch forward, the weight of the suitcase in front would push you back, then gravity and your weight would cause the suitcase in back of you to topple sideways and you would end up on your back with the front suitcase on top of you.
But this is just quibbling. We don’t know the exact physics of how it happened and we shouldn’t care. The bottom line is that the girl somehow fell and couldn’t get up immediately, and was embarrassed in front of a large group of men who, for whatever reason, chose not to help her. And that was wrong.