When either pendulum swings beyond a minor angle specific to each’s length and mass, both pendulums quite suddenly interact in a manner called chaotic motion. Not quite as the name implies, chaotic motion can be predicted, but only by knowing the exact triggering circumstances at virtually atomic level and via hugely complex mathematical equations. Let us just say for now that the behaviour of a double pendulum is random-like, but not random.
A tow vehicle and trailer, when subject to external forces, behave like a horizontal double pendulum. By being on the ground, due to the force of gravity on the rig’s tyres’ footprints, their friction on the road surface dampens the action. The effect is not as intense as the vertical pendulum, but the principle is the same: yawing