Classically, mathematically, the answer is conservation of angular momentum. As matter falls on to the body the body needs to rotate faster to conserve the ener

Classically, mathematically, the answer is conservation of angular momentum. As matter falls on to the body the body needs to rotate faster to conserve the energy of the system. A rotation happens if there is even a tiny asymmetry in the accreting matter. In the case of our own Moon, one side is more massive than the other and over the course of millions of years became tidally locked so that the heavier side always points towards the Earth.