Why do helicopters spin around when they crash?

Helicopters do not spin around when they crash, but they rotate rapidly due to dissymmetry of lift on the rotor disk caused by: rotor dissymmetry (main and tail, rotor RPM differences); wind (ground effect wind, translational lift); or wake from another helicopter/obstacles. As the craft descends rapidly under power into its vortex ring state the combination of forces on the fuselage will induce a roll moment. Without sufficient altitude to correct or counter the descent there's insufficient blade speed and lift to regain control of direction; hence, rapid, uncontrolled and possibly fatal rotational forces occur at ground strike.