  Solar eruptions often show the rotation of filaments, which is a
manifestation of the rotation of erupting magnetic flux rope (MFR). Such a
rotation of MFR can be induced by either the torque exerted by a background
shear-field component (which is an external cause) or the relaxation of the
magnetic twist of the MFR (an internal cause). For a given chirality of the
erupting field, both the external and internal drivers cause the same rotation
direction. Therefore, it remains elusive from direct observations which
mechanism yields the dominant contribution to the rotation. In this paper, we
exploit a full MHD simulation of solar eruption by tether-cutting magnetic
reconnection to study the mechanism of MFR rotation. In the simulation, the
MFR's height-rotation profile suggests that the force by the external
shear-field component is a dominant contributor to the rotation. Furthermore,
the torque analysis confirms that it is also the only factor in driving the
counterclockwise rotation. On the contrary, the Lorentz torque inside the MFR
makes a negative effect on this counterclockwise rotation.
