Why must a driver leave more following distance with a loaded tank than with most other vehicles?

A loaded tank needs extra stopping room for several reasons. Its high center of gravity makes hard maneuvers risky, surge can push the rig forward during braking, and the cargo weight lengthens the distance required to come to a complete stop. Wet or slippery roads make all of this worse. The manual therefore stresses anticipating stops far ahead, braking gradually, and keeping a generous cushion of space in front so you are never forced into a sudden, hard stop that could trigger a skid or surge-related loss of control.