Why does the AAMVA manual warn that strong winds are a particular hazard for school buses?

AAMVA CDL Manual, Section 10.7.2 (Driving in High Winds), p. 10-11

Many drivers assume a large, heavy vehicle like a school bus is immune to wind effects, but the manual explicitly warns otherwise because of the bus's flat-sided shape. Understanding the sail effect is what motivates the specific precautions, slowing down or pulling over, that follow.