According to the CDL manual, what is the main danger of a load with a high center of gravity (cargo piled up high or heavy items loaded on top), especially in curves or during a sudden swerve?

49 CFR 383.111(a)(16); AAMVA CDL Manual (nhcdm.pdf) Section 3.2.3, p. 3-2

Top-heavy loads behave very differently from low, well-balanced ones, especially when a driver has to swerve suddenly to avoid a hazard. A load piled too high can cause a rollover in a curve that a lower, properly loaded vehicle would have handled safely.