this is a classic fatigue fracture, caused by a cyclic/reversed bending load superimposed on a static tension load. The cyclic load is caused by the reversed bending; the neutral axis shows clearly as a straight line running diagonally across the center of the part. the fact that it is a straight line demonstrates that the torsion component was not the dominant load.
the initiator of the fracture was the sharp tip of the cut thread in the part and the reversed cyclic loads caused the fatigue fracture crack to develop on both sides of the bending neutral axis. Finally, note that the cross-section of the final failure caused by overload (the area defined by the shear failure zone along the diagonal line as defined above) is small compared to the unfractured cross-section of the part; this demonstrates that the loads originally imposed on the part were small compared to the tensile strength of the part itself as designed. The conclusion therefore is that this part was not designed for fatigue service; it should have had the threads pressed into it with a set of roller die rather than having them cut on a lathe or with a threading die.