Why do passengers in the bus fall backward when the bus begins to move suddenly?

Initially both the bus and the passengers are at the state of rest. The force applied on the bus by its engine makes it move. A sudden movement results in a change in state of motion of the bus as well as the passenger's feet in contact with it. But rest of the passenger's body opposes this change in state of motion due to its inertia and tries to remain where it was. Hence the passengers fall backward when the bus begins to move suddenly.