Question

Why a minibus is more likely to topple over when the roof rack is heavily loaded than when the roof rack is empty?

Answer 1

Try a turn, look at
Ac = v^2/R
needs side force from road at bottom
moment about CG is balanced by light force up on inner tires and heavy force up on outer tires. The higher the CG, the longer that moment arm.
When v^2/r is high enough, inner tires lift off road.

Answer 2

this is when the roof rack is heavily loaded because of the tilting bus is increasingly smaller. as the base size reduces in size, the action line of the weight of the bus moves closer to the edge about which the bus is tilted.

Answer 3

Because the center of gravity is higher on the roof thus making the bus unstable