Why must a driver be extremely cautious, especially when starting and stopping, in an un-baffled tank vehicle?

49 CFR 383.119(c); AAMVA CDL Manual Section 8.2.5, "Un-baffled Tanks," p.8-8-2

A strong, uncontrolled surge at the exact moment a driver is braking or accelerating is one of the more dangerous handling quirks of tank vehicles. Being extremely cautious at starts and stops, as the manual instructs, is a concrete habit that directly reduces the chance of the surge overpowering the driver's control of the vehicle.