I may never know what actually causes lead/acid batteries to explode.
In so many years of tending these things, I have seen a single battery explode when a failure caused a massive discharge.
All of the other explosions I have seen were two batteries close strapped together. A vast majority of these were when a new battery was added to the bank, and it was a tossup whether the new or the old when up. Owners always hoped it was new, because at lease that had some warranty to use. The chances of a battery explosion go way down as they are separated as in diesel pickups, but I have seen two there also.
Matt