How does a rotating disk form around a protostar?

A rotating disk, often called an accretion disk, forms around a protostar when the infalling matter possesses significant angular momentum.1

Because this matter has sufficient rotational motion, it cannot fall directly inward toward the central object in a straight line. Instead, the material flattens into a disk shape in the directions perpendicular to the protostar's rotation axis, as the rotation resists the direct inflow of gas and dust. In the directions parallel to the rotation axis, the matter contracts toward the equatorial plane until the internal thermal pressure roughly balances the gravitational force.1

As the material within this disk spirals inward, it must lose energy and angular momentum, a process driven by turbulence and viscosity within the disk. This turbulence is thought to be generated by the amplification of magnetic fields due to the disk's rotation, which facilitates the inward drift of matter while transporting excess angular momentum outward.1