A 'leading' power factor indicates that the load current is leading the supply voltage. Since power factor is the cosine of the angle by which the load current either leads or lags the supply voltage (i.e. the load's phase angle), a cosine of 0 corresponds to a phase angle of 90 degrees.

So, a power factor of 0 (leading) indicates that the load current is leading the supply voltage by 90 degrees, which means that the load must be purely capacitive.

When its load has a leading power factor.

due to decrease in power factor

In order to draw the phase diagram for transformer operating at load with lagging PF and leading PF, you will need to know the equation for the transformer being load free. This constant will help you with the load bearing equation of Np/Ns=Vp/Vs=Is/Ip.

Because in the leading load or capacitive load the armature reaction is magnetizing.Here the stator flux adds up rotor flux and so the net flux of alternator increases and the net flux produces alternator's voltage.Hence the alternator's voltage rises.