Why does the AAMVA CDL Manual instruct students not to stop or go back for a dropped object near the bus?

AAMVA CDL Manual, Section 10.2.5 – Special Dangers of Loading and Unloading, p. 10-5 (July 2017 revision); 49 CFR §383.123(a)(1)

Understanding the "why" behind this rule matters because it isn't about being tidy or on schedule - it's about the seconds a driver can lose sight of a child near a multi-ton vehicle. That window, however brief, is when a bus moving even a short distance can cause a fatality, which is why the manual treats a simple dropped object as a real safety issue.