In-service gas-insulated switchgears (GIS) often experience arc discharge phenomena. Existing studies generally attribute this to the rise in enclosure ground potential caused by the very fast transient overvoltage (VFTO) generated during switching operations, without conducting in-depth theoretical analysis. This paper theoretically derives the mathematical expressions for the transient current and its rate of change during the switching of unloaded busbars in GIS, as well as formulas for calculating the transient voltage induced at the insulation partition of the GIS enclosure by the transient current. For GIS rated from 126 kV to 1100 kV, the rate of change of charging and discharging currents during switchgear switching can reach as high as 10⁶ and 10⁹ A/s, respectively. Such high rate of change in transient current can induce significant transient voltage at the insulation partition of the GIS enclosure. Multiple re-breakdowns between switch contacts during operation can induce secondary VFTO at the insulation partition, leading to air-gap breakdown and arc discharge. Finally, this paper briefly describes observed various arc discharge phenomena in the field and proposes an effective mitigation measure: reducing the loop area that includes the insulation partition of the enclosure can effectively lower secondary VFTO on both sides of the insulation partition, fundamentally eliminating arc discharge.