Hi Isaac, sorry to hear about the pack, I know how it feels.
I've had my master reset to 0 cells once when I was having a problem with slave 25. The slave was behaving erratically and I finally tracked it down to a poor connection between the 12F683 and it's socket, reinserted it and its been fine since. But when it was playing up I did have to drive 10 miles with the alarm going and it did cause the cells to reset to 0 in the master. I reset the cells to 48 in the menu and its been fine since. I suspect that the misbehaving cell sending garbage data somehow reset the master, you might have had the same thing, low cells causing garbage to be sent back to the master, causing it to reset. So I think the cells being reset to 0 was probably a result of the pack being over discharged and not very likely that it was the cause.
The real question is what caused the pack to get discharged. Since it's all the cells, it has to be something that affected the whole pack. A fault in a single slave can cause it to turn on a load resistor and start discharging a cell, but that would only affect that 1 cell, not the pack. Could the cold have affected the slaves (and the cells themselves)? Electronic components are sold in different grades, each with different operating temperature ranges. Components used outside their rated range might not behave as expected. Testing would be required. A resistor ladder could be used to simulate a pack and test the slave operation in cold temperatures.
Or something else drained the pack, Peter has killed a couple of packs this way.
It's important to make sure everything gets disconnected like dc-dc converters when storing an ev for extended periods. In my Matiz I have a contactor in the middle of the pack that opens whenever I switch off the ignition. Hope you get to the bottom of what went wrong.