This is not completely unheard of. It can occur when a battery develops a weak spot in the lead plate grid inside the battery. Typically this can happen in an older battery because the plates have sloughed away a quantity of material over time.
What happens next is that sections of plate material can break off leaving a very small "filament" of plate that is incapable of carrying the load of the starter the next time the engine is cranked. The filament of metal burns out and the resulting flash arc ignites the explosive mixture of gases that are present in batteries.
A battery that is low of water/electrolyte will produce the most violent bang because there is more available space to fill up with the gases.
The short version is: Defective battery.

"what should I do now?" Get some baking soda and bucket of water and wash all the battery acid off. Then go to sears and by 24.95 battery. Say WHAT!! I'm not buying a cheap sears battery for my JD. Sears or JD don't make batteries and it only takes about 90 CCAs if that to crank and start you small LT engine. so the Cheapest 160 CCA battery has plenty of power for LT engines less than 20 HP.

IMO You was very lucky to get 6 to 8 years out of battery I replace mine every four year if they are bad or not in all my equipment including cars, any battery over 4 years old is ready to go and it usually at the wrong time when it does.