My first guess would be that the battery electrolyte level was not checked BEFORE the charger was placed on the battery.
The electrolyte was below the top of the lead plates. The battery charger was placed on the end life cycle battery and generated (outgassed) large amounts of hydrogen and oxygen.
When the internal combustion engine was started the battery plates warped by the rapid electrical discharge draw from the electrical starter motor and NOT being submerged in electrolyte produced an electrical spark in a hydrogen / oxygen gas environment.
Hydrogen and Oxygen on an atomic weight basis; releases more energy than nitroglycerin .... BANG!