[Jaydee]

Are both end of the battery case bowing out? Plates are sulphated. If you have any plates that are almost touching, they will create a resistance, especially when charging, therefore creating a localised heat, boiling the fluid.
i wotk for roadside service, and batteries fail for all sorts of reason. Some die instantly, some slowly. The one you have to watch out for, is the open circuit battery. It will destroy computers etc.
JD