Synchronous Motor Leading Power Factor
The operation of a synchronous motor with a leading power factor is explained by analyzing the phasor diagram. It has a leading power factor when the excitation is higher than the excitation required for operation with a unity power factor. That is the definition of "overexcited." Leading power factor operation is explained mathematically using the phasors derived from the equivalent circuit. There  may not be an explanation that is intuitively satisfying.