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I know that fault levels are based on pre-fault voltage and that this relationship is proportional.

I = V / X

where X is the system impedance as seen at the fault.

Reactive power too is strongly related to voltage, such that increasing reactive power tends to increase the local voltage in an area.

Combining those two effects, increasing the reactive power in some instances will increase your pre-fault voltage. And if you are running PSS/E ASCC analysis by default this higher prefault voltage will give a linearly higher fault level.

That is, increasing your pre-fault voltage by 5% can increase your fault level by 5%

To avoid the reliance on pre-fault voltage The Australian Standard for fault calculations recommends that all simulations be based on a nominal 1 p.u pre-fault voltage (no variance) and a fudge factor of 1.1 be applied to the final result to account for the fact that power lines are often run at higher than nominal 1 p.u voltage.

psspy.ascc_2(sid=1,
             status1=1,      # line to ground faults.
             status4=1,      # impose nominal 1.0 p.u prefault voltage.
             # ... rest of arguments.
)

or using the pssarrays module

short_circuits = pssarrays.ascc_currents(
             sid=1,
             fltlg=1,        # line to ground faults.
             flat=1)         # impose nominal 1.0 p.u prefault voltage.