Winter temps don't kill all bacteria. Many just slow down their metabolism. It's only if the water in their cells freezes over that there would be a problem so bacteria living under the ice could survive. There are even some bacteria that survive in sea ice by secreting their own "anti-freeze" (essentially any high concentration of ions that can fully saturate the water in their cells will do).

It's not the temps that are the mystery so much as the survival with chlorine. The breakdown of cyanuric acid is shown in this link where you have to click on some links to see that the end product is not only carbon dioxide, but ammonia. waterbear mentioned this in another post somewhere. Anyway, so far in my own pool I haven't found any decline in CYA over the winter and I kept the FC level up the whole time. So, at best, we have inconsistent results.

The chlorine level still declines in cold water; it's just slow. In my pool with water temps around 50F and with a pool cover, the chlorine drops barely 1 ppm FC in 2 weeks at a level of around 4 ppm FC. In the summer at water temps of 88F I lose about 1 ppm FC every 2-3 days and that's with an opaque pool cover. If you never added chlorine after your winter closing, I can certainly imagine the level dropping to very low values after several months, especially if there was still some sunlight on the pool.

Richard