Arcing is not possible in the senerio.  The voltages are WAY to low. You would need hundreds of volts or even thousands to jump even a small amount.

http://en.wikipedia.org/wiki/Paschen's_law

This was current runaway.  These chips are most likely a "chained" design used to obtain maximum efficiency.  If one chip in the series "chain" shorts internally, it can

cause a great increase in current through the other chips in the chain thus causing the temperature to rise suddenly and dramatically.  This would literally set them on fire!

Somewhere on here I have seen a picture of a heavily burned "one string miner" that used Bitfury chips.  These use 14 chips and run directly off the 12Vdc feed in series.  

So if one ship shorts, it starts an overcurrent condition that very quickly pops all of the others.