Trying to keep it short and simple, so ask for more details if you want.
All AC systems have a natural capacitive coupling to ground. In a balanced system the associated leakage current cancels simialr to the way the balanced phase currents cancel. But if one of the phase conductors is grounded then there is no canceling of the charging current.
In a low resistance grounded wye system the overcurrent protective devices should operate when a phase goes to ground preventing further problems. However in high-resistance grounded wye systems the OCPD will not open causing the charging currents form the other two phases to increase by a factor of 3 (this is caused by the change of the phase angles and the increased voltage to ground).
The charging current in a high resistance ground system is minimal so even a 3 times increase is not usually a safety hazard. The primary reason we are concerned at all has to do with the settings and operation of protective realys and other ground monitoring equipment.
