Initially, the passengers and the bus both are moving in same direction. When the bus suddenly takes a sharp turn, the lower portion of passengers which is contact with the bus tends to move in that direction but the upper portion of the passengers throw an outside. This happens due to the INERTIA OF DIRECTION.
When the running bus stops suddenly, the passengers are jerked forward because due to inertia of motion the passengers tend to remain in their state of motion even though the bus has come to rest. Q.
When a bus abruptly starts moving, the passengers tend to slide backward due to inertia, as the passenger's inertia tends to oppose the bus's forward motion. Hence, the passenger tends to fall back when the bus accelerates forward.
When a person alights from the bus, his body would try to continue moving in the direction the bus was moving due to inertia. But as soon as both the feet touches the ground, the lower part of the body comes to rest while the upper part gets carried away under the effect of inertia resulting into his forward fall.
When a bus starts suddenly,its passengers tend to fall backwards because due to their inertia, the passengers tend to remain in a state of rest even when the bus starts moving.
You know you are riding on a bus as a passenger, and should make sure you can grab something to steady yourself as you try to find a seat. The driver is expected not to wait until passengers are all seated before starting up again!