OK, then it's the condition of the connection between the battery terminal and the conductor that bolts on to it and goes to the fusible link box. It carries the juice for the whole car except the starter. Bet the brake lights at least were on? Maybe the fan? For sure the starter solenoid was drawing juice. Enough current to pop a good spark. Dielectric grease is your friend on any bolted electrical connection, 'specially around batteries. Clean up the contact surfaces with a wire brush and grease 'em up and bolt them tight.

Not responsible for advice not taken...