A trailer jackknife, where the trailer's wheels lock up and the trailer swings around, is MOST likely to happen when the trailer is:

AAMVA CDL Manual, Section 6.1.5 "Prevent Trailer Skids," p. 6-2

Trailer jackknifes happen fast and with little warning, especially when a rig is running empty or lightly loaded, a common condition when deadheading to a pickup. Knowing which loading condition raises the risk helps a driver brake and follow more conservatively exactly when the danger is highest.