Happens a lot on golf cart batteries if you keep them around too long... 5 years is about too long...
One of the cells can short out and the heat generated will cause it to spew acid and hydrogen, they usually will cook to an open after shorting, especially if there is a charger on them... If it all happens just right a spark can ignite the hydrogen and that will pop the case...
Mix up some baking soda & water to flush out the battery box and cover... new battery & you should be good for a long time... :smile:KennyV