My guess is that some solder got onto both posts of the Deans connector. I've almost made this mistake myself when I've been in a hurry and soldering up deans.

For the fire to occur, basically a short occurred somewhere. Either in the charger, the connections, the wire (if it had an exposed area due to cut insulation), or the battery itself.

I'm not familiar with that charger, but I think you can rule out the battery since it's not a pile of ash. So my guess is either a connection, or the wire was damaged. (Also, I've seen weird things happen when charging on a metal table.)

Paul