Question

why do passenger fall back ward direction when the bus start moving from rest of position

Answer 1

It is due to Newton's first law of motion also known as law of inertia.
It states the body wants to remain at rest when in rest and in motion when already in motion.
So when the bus starts moving our body is at rest wants to be at rest.
As the bus progresses forward our body resists this change and tries to remain at the original position which we perceive as the body falling backward.
The same is also responsible for the body falling forward when the bus stops and the body tilting left and right at sharp turns.

Answer 2

this is because of inertia.we are in the position of rest and when the vehicles starts moving suddenly our lower body continoues to be in the same position and so we tend to fall back when it starts suddenly