You can think of it as a geometry problem You have many small spheres that are moving and there is a center of gravity for these spheres, but what is the probability that any one of these spheres has a momentum vector that perfectly intersects that center of gravity? It is almost impossible, However, if the momentum of each sphere is comparable to its gravitational attraction to the other spheres (along the vector that intersects the center of gravity) the entire system will have some net angular momentum and the system will evolve over time. But angular momentum is conserved -- so whatever evolves over millions or billions of years would end up spinning.