The answer given by Mr Hamid is quite absorbing, but it does not give the full idea why the reactance of the machine varies from sub transient to transient to steady state. In my opinion, a fault can be defined as a sudden change of impedance of a circuit. Hence, sub transient or transient must occur during load change also. The answer to why they occur lies in the fact that flux takes a finite time to respond to a change of current depending on the magnetic behavior of the core material. Hence, the ratio of change of flux to change of current (i.e. inductance) in the circuit varies during the first few cycles leading to sub transient reactance. To counter balance the stator flux so set up during the sub transient phase, the field circuit again needs finite time leading to transient behavior. After transient phase, the steady state is achieved. If there is any error in my understanding, I request you to please correct me.