Why does rotor of synchoronous generator accelerate during faults?
The electrical power out of the machine is P = ((VG x VR)/X)sin(d) where VG is the generator voltage, and VR is receiving bus voltage. X is the reactance between the two. d is the angle difference between the two voltages.  We use per unit typically when solving, so, at normal conditions both voltages are near 1.0.   When a fault occurs in the nearby system VR is depressed, so the P that can be electrically moved out if the machine is reduced proportionately. So, the excess power being put into the machine (mechanical torque) goes into accelerating the rotor.

I would recommend Kimbarks 3 volumes as they are classics and volume one is the introduction you need to this topic.  

Edward Kimbark
Power System Stability, Volumes I, II, III, 3 Volume Set

regards,
russ