I suppose an explosion could occur to a battery that did not have proper venting, or the owner disabled it (removed venting hose, relocated battery to a non-vented area, etc.).
Closest I had to a battery explosion was when my regulator on my 300E died, and the alternator was pushing 17.5V to the battery!!!
So the battery started venting...
...you could see (and smell) the acid vapors emanating from under the hood, even when closed!