Most likely an arcing ground fault, that caused a secondary arcing ground fault on an adjacent phase, of the supply bus bars in the MCC.
I agree with Don. Any problem upstream wouldn't initiate an arcing problem in the MCC, considering the normal clearances and insulation associated with MCC bus bars. It would take one heck of a high voltage spike to initiate the fault. It might be possible that the 13200 momentarily popped over to the load side and initiated the fault, so George's suggestion of a megger test makes sense. Still need to know if there are any other loads on the transformer, and distance to MCC, before guessing about weird circumstances. If there was a better chance for an momentary over-voltage to fault to ground in some other gear than in the MCC, but it didn't, then most likely it was just something local to the MCC.
As Don said, maybe some critter pushed a wire into the bus. You could look at the gear and see if the dust or some loose wire might have been involved..tho I suppose you already did that.
Scary problem. I certainly wouldn't want to re-energize with out meggering everything.
