I agree of the possability of 2 breakers feeding the same circuit.
Did you short the circuit hot to the Nutral or the equipment ground? If it was hot to EG then a question of the equipment ground integerty comes to mind. How far does the fault current need to travel to get to the incoming service? Is the EG conduit only? Could be loose connections at boxes or couplings creating resistance on the branch circuit or between a sub panel and service. You could look at the service equipment grounding.
The ideal suretest I never used myself but I believe it tests the EG for impedance.
