Earth's changing motion (or acceleration) around the Sun is caused by the same thing that causes any object to accelerate, an .

A change in direction

Explanation:

Earth's changing motion/ acceleration around the sun is caused by the change in direction of the earth around its orbits. the change in direction of an objects leads the object to accelerate. and

Acceleration of an object depends solely on the change in the velocity of the object , while velocity is a vector quantity which is caused by the change in displacement of object over a period of time

v = d / t      while A = V / t