Is it possible that what is being reported as a 'wire' could actually be two wires sleeved together and forming the flying lead from the battery pack?
If that is the case then pinching that may be all you need to short the battery. Trouble with that is I can't imagine that there wasn't a fuse within the battery, so it should still be protected against a short circuit.
Assuming the cause was thermal runaway in the lithium battery, and I can't think how else there was enough fuel to keep a fire going long enough to do the damage, then I think we are left with a failure that somehow didn't draw a high enough current to blow the fuse but was high enough to heat the battery fairly significantly.
That, combined with a very hot day, a closed up airplane and the ELT located in the crown could easily have got the battery temperature to over 60 deg C which is all it takes apparently!