Why did the passenger lean forward?

Why did the passenger lean forward?

When a sudden brake is applied in a moving bus the passengers lean forward because of the inertia of motion. When a bus is in motion, the passenger of the bus is also at the same speed as the bus. 

When the bus stops suddenly, the lower part of the body also becomes stationary with the bus. But the upper part of the passenger's body moves forward due to inertia of motion. That's why the passenger leaned forward.