Answer:
Inertia takes the answer for this question.
Explanation:
When a person is inside a vehicle, the person would ever so slightly try to lean back into the seat as the vehicle accelerates, preventing itself from being in an uncomfortable position. When the vehicle stops, however, the person would not have fast enough reflexes to adjust to the amount of power he/she has to exert to be in the same position, therefore, the person would lean forward, slightly, or dramatically (depends on the exaggeration of the crash) which therefore answers the question of why a person would be thrown forward in their seats when a speeding bus suddenly, and abruptly stops.
