Question

explain why bus start suddenly the men
falling backward

Answer 1

The reverse happened when the bus or train started suddenly, then that time our body fall backward because lower body stick with the bus so it goes forward while the upper part tries to remain in rest due to inertia 

Answer 2

this happens because of inertia , our body is initially at rest but suddenly when the bus starts moving , our feet that are in contact with the bus , tend to move forward but our body remains stiff and falls backward .

hope it helps .