The thing you need to remember with lead-acid batteries is that they give off hydrogen gas as they cycle. EXTREMELY explosive hydrogen. This is why even if you hook up a battery correctly it can explode if you make the last connection at the battery, arc ignites the gas=explosion.
Even if you make the last connection at a remote ground then wiggle the battery cable, you make arcs at the battery once again=explosion.
When you make reverse connections you've doubled the voltage in a short circut which eventually cause the battery/batteries to explode from heat buildup. HOWEVER if you start with reversed connection you get a very large arc and ignite the hydrogen coming off the battery.
It's not JUST stupid or inexperience that causes battries to explode.