The luggage placed on the bus roof is not fixed to the bus. When the bus starts suddenly, the luggage tends to remain in the state of rest due to the inertia of rest.
As a result, the luggage might fall towards the backside of the bus. When a moving bus stops suddenly, the luggage continues to remain in the state of motion due to the inertia of motion.
As a result, the luggage might slide towards the front side of the bus and fall. To prevent fall of the luggage from the roof of the bus, it is advised to tie the luggage with a rope.