We've all been there.
You're late for a flight. Your roller suitcase hits a bump, and suddenly it starts wobbling from one side to the other. You slow down, to get it under control, and it only seems to get worse — until you have a full-on baggage meltdown. As the BBC puts it: "Luggage rocks from side-to-side until it falls over, or it reaches a regular side-to-side swing."
If this has happened to you and you felt at the mercy of forces greater than yourself ... you weren't wrong. That was actually the case.
French scientists investigating the phenomenon blame it on "the coupling between the translational motion and the rotational motion of the suitcase" — a problem shared with any object maneuvered on two wheels connected along a single rod. "The study would be the same for any trolley with two wheels or blades," one of the scientists told the BBC.
The solution is easier to explain than the underlying problem: go faster. Slowing down actually tends to exaggerate the wobbles. You can also hold the handle closer to the ground, reducing the swing between the two sides.
Or, y'know, just get a duffel.