Lead/acid batteries, actually, all batteries, sluff off plate material as they go through their usable life, and the sediment eventually builds up in the bottom of the case to where it shorts out a couple or more plates, effectively "killing" the battery. This process may go along faster or slower, depending on the factors involved in the charge rate/level/voltage, the environment it lives in, and the usage. Yours was probably just on the verge, and you hit a bump or some such and jarred it enough and it finally shorted the plates, so when you shut it off, it was done for. The big batteries we had in our plants were up to 120 cells, and were housed in glass jars, usually in a separate battery room. You could see the sluffing taking place, and we had to measure it and keep track so the battery shop could replace cells as needed. At night, when things were really quiet, you could hear the cells percolating and making small "burps" and bubbles.