After seeing some of the old UPS batteries bulging to the point we had to cut them out of their enclosures, cracking the battery cases, etc., I've generally just classed all batteries as things to get out of the house/business as fast as possible after they're out of service.
AGM batteries are packed with fiberglass, so when oxide starts building up on the plates because of improper charging or electrolyte drying up they start pushing on the case.
