Lead acid batteries do 'gas' when being charged producing a hydrogen gas by-product. This hydrogen gas is lighter than the air around you so it will accumulate in the overhead nooks and cranny's of the compartment. Securing the charger may have not been enough. They should have exchanged the compartment atmosphere as per IAW before firing up the cutting torch.
Unfortunetely when that someone, sans qualified electrician, flicked the switch without checking the circuit he caused one big bang. Hollywood did the rest.
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