Why does an empty or lightly loaded combination vehicle have poor traction when braking hard?

AAMVA CDL Manual, Section 6.1.3 'Brake Early,' p. 6-1

A driver who does not understand this mechanism may brake just as hard whether loaded or empty, increasing the risk of locking the wheels and triggering a trailer jackknife. Braking early and controlling speed matters in both conditions, but the risk of wheel lockup is specifically higher when lightly loaded.