The shaft is being displaced causing the seal to hit something as it rotates or to cause the rotating face to run off of the stationary face. The rotating assembly is out of dynamic balance. The shaft is bent. There is misalignment between the motor and the pump. Pipe Strain is twisting the pump stuffing box.
Heat causes expansion and that always opens the possibility for rubbing or wear.
Cavitation, slip stick, harmonic vibration, bad bearings or some other form of vibration is causing excessive movement of the shaft. The pump designed with sleeve or babbitted bearings and shaft movement is excessive. The seal face is being distorted by either temperature or pressure. The product is vaporizing between the seal faces causing the faces to blow apart. An environmental control has failed.