Thinking further about it an internal short(s) to a bad batch of batteries would have a 10 or 8 remaining voltage battery charged as a 12 volt, quickly putting paid to any good remaining cells.
I've certainly seen this happen on a boat with one cell shorting in a battery that's part of a bank. The others then try to charge the faulty one resulting in heat, fumes and battery case expanding. Thankfully the easily identified faulty one was isolated, although if left an explosion would have been a possibility
