What causes batteries to explode?

Answer 1

Heat basically. When you push a high current into a battery, charging it, some of that will turn into heat. And enough heat will pretty much cause the battery to boil and the expanding gases causes it to burst open. A high discharge will do the same.

Answer 2

Two things can cause a battery to explode. The battery generates hydrogen and oxygen gas, which is explosive. A spark can set it off. This is why, when you jump start a car, the "last" connection must be to an engine ground connection - away from the battery - and the "first" disconnection must be at the same point. More likely, however, is that the battery was short circuited, perhaps with a wrench that fell across the terminals. It might also have a internal short circuit, perhaps caused by buckling which might be caused by a bad starter. The alternator is typically not a culprit, because there is usually a fusible link in the high current wire going to the alternator.
It was probably leaking gases that ignited when somthing thing sparked after the key was turned. Specifically, hydrogen -- picture the Hindenburg in miniature, only faster. Assume you know for sure it was the battery though and not say a fuel leak or anything, seems like you'd need a good bit of combustible to get that effect. I have seen battery's explode from overcharging, which causes copious amounts of hydrogen to be generated (just submerge a battery in "pure" water and overcharge it), you will be amazed at how much hydrogen is generated you can see it roil the water, believe me it's alot lol. Then "any" point of ignition and "Big BOOM". It's not pretty either, with all that acid all over the place ;-( Be VERY careful when charging a battery and make sure you do not overcharge it. Also be very carefull when "Jump Starting" a car, for the same reason. TommyTrouble