The HDD drill pipe features a Double Shouldered connection. The box's surface connects with the pin's shoulder to establish the Exterior torque shoulder, while the pin's tip connects with the box's rear to create the internal torque shoulder.
When you apply over torque to the connection, it causes the material to shift. The outcome of over torquing is the box expanding at the connection point, as the material attempts to pass over the raised part of the pin, and the pin's tip getting deformed as it forcefully collides with the back of the box.