As mentioned the pack could have been damaged in a crash or dropped. The charger could have been set incorrectly also.
I must admit I've never witnessed or even heard a story of anyone(at the fields I fly) experiencing a fire while charging and a lot of folks fly electric and use some large packs.  I believe if you take care of the packs, handle them, charge and maintain them properly and in a safe manner  and stay away from the low quality packs you will have no problems.