What phenomenon causes surrounding material to flatten into a rotating disk around a contracting protostar?

Answer

Conservation of angular momentum

As the cloud material collapses inward, its initial rotation is conserved, forcing the material into a flattened, spinning accretion disk perpendicular to the axis of collapse.

What phenomenon causes surrounding material to flatten into a rotating disk around a contracting protostar?
astronomyforcegravitystar formationinterstellar cloud