Before 1950 power system were often without neutral grounding. Such system had repeated arcing grounds, insulation failure and difficult earth fault protection.
Every phase has inherent distributed capacitance with respect to earth. If earth fault occurs on phase B, the distributed capacitance discharges through the fault. The capacitance again gets charged and gets discharged. Because of this sever voltage oscillation is reached in healthy phases.
These voltage oscillation causes stress on insulation of connected equipment.