Answer:
This happens because of inertia.
Step-by-step explanation:
Inertia is the ability of a body to remain in its state of rest or relative movement, while an external force is not acting on it.
It is when a body objects to modifying its state of relative rest or movement, including the speed and direction in which this happens.
When a car accelerates, a cup of coffee sitting in a cup-holder spills out onto the rear seats, because it has left its state of rest due to the acceleration force of the car.