During my youth I was working at a mobile fast food stand
Once we needed to take the highway to jump in on a job and my boss just jokingly told me to look into the mirror
The fucking fast food trailer was jumping a meter to each side

He was having quite some fun provoking it by accelerating, because it started around a specific speed
So in that case it was probably an aerodynamic issue - at least it seemed like that

We weren't going downhill or uphill, and the only difference was our speed when the jumping started