What causes the dangerous 'crack-the-whip' effect in combination vehicles?

AAMVA CDL Manual, Section 6.1.2 'Steer Gently,' p. 6-1

Many trailer-overturn accidents happen even though the tractor itself never loses control, because the trailer whips outward during a sudden steering move. Recognizing that the cause is the steering input, not road conditions or equipment defects, reinforces why gentle, smooth steering is the fix.