A cargo that is not secured will shift when the forces as a result of acceleration, deceleration or change of direction are bigger than the frictional force between the cargo and the loading floor.
The magnitude of the frictional force depends on these forces caused by acceleration, deceleration and/or change in direction, on the gravitational force acting on the mass (weight) of the cargo and the material of which cargo and loading platform are made.
The moment at which the cargo starts to shift, however, is not dependent on the mass or weight of the cargo because the energy stored in a heavy object that moves (the potential energy) is larger than that of a light object.
At acceleration, deceleration or change of direction, the mass force is proportional to the weight of the cargo. A heavy and a light load will, therefore, start to move at the same time, as shown in this 20 second video:
After watching this video I am sure that you agree that the thought that “if an object is heavy enough it does not need to be secured” can, therefore, be thrown directly in the trash bin!