A natural force that cause rocks to fall or roll downhill?

Answer 1

Wind and rain erosion causes rocks to become dislodged will let it fall or roll to the bottom.

Wind and rain erosion do cause rocks to roll to the bottom of a hill but more importantly gravity does. Gravity is the main reason :)

Answer 2

Weathering and erosion cause rocks to break apart and be transported from one place to another, and the main agents of both are water, ice, wind, and in the case of this question, the main one would be the force of gravity.