Why does the AAMVA manual say that starting and stopping a bus should be as smooth as possible?

49 CFR 383.117(a)(5), proper braking procedures; AAMVA CDL Manual Section 4.3.2 (p.4-3)

Smooth braking and acceleration is one of the few passenger-specific skills with a direct, named federal knowledge-test topic. Expect questions tying "why" smoothness matters directly back to preventing rider falls.