Won't let me see your pics but I assume the batteries are swollen.( I have just joined the forum but system reports I don't have the rights to view the pics?)
There are numerous reasons why this can happen -
Batteries shorted at terminals possibly by 'salts' building up on casing
Cell failure within the battery
Mostly nowadays batteries are sealed if not check electrolyte levels and SG
Insufficient cooling of the batteries
Incorrect charging current
Failed battery within the circuit resulting in other batteries having to make up for the dead battery putting excessive strain on them.