I have seen far to many battery explosions. One right under my sit down. Yep, on a golf cart.
I am an electronic tech and diagnoses battery issues thousands of times. Always wear some sort of eye protection around a battery. One spark or even one that goes bezerk internally. Usually an old one or one that has been overcharged. A battery temperature sensor can turn a charger into a boiler.
Folks are mentioning low water levels and that can be a clue. For now if those batteries have any age on them it would be a good thing to get rid of them. You need to pull out all the batteries (after lots of pictures of course), and clean everything with warm water and lots of baking soda to get rid of the acid that will eat everything in sight and beyond.