Exploding lead acid battery
simulate this circuit &ndash; Schematic created using CircuitLab

You've described the above situation. Ri * 3 represents the internal resistances of all three non-shorted batteries. If the number I gave is correct, then 2.4kW will get dissipated somewhere when the switch is closed. If the sizes or constructions of the batteries are different then most of that power could be dissipated in a single battery. That could cause the electrolyte to boil and the battery to vent. I don't think it should explode but maybe using an external battery caused more gas production than the vents were capable of dealing with or maybe the vent was plugged, or it was a badly made SLA battery. 

Of course reverse connection would be similar but much worse with 3x the voltage and 22kW of dissipation.