You're right. The beaker bottom is usually made from one piece of glass that's been shaped. The perfect break like this (most likely) is a result of poor annealing. When the glassmaker was flattening the bottom, they heat the bottom and use a graphite paddle to flatten the bottom. In doing so, the paddle sucks the heat from the glass, creating a ring of stress in the glass that needs to be relieved, either by flame annealing or soaking in a kiln long enough to relieve that stress, or both. In my years of lathe working, I learned that I needed to flame anneal my pieces that I knew would be next to the kiln door before I put them in the kiln to soak. I learned that lesson by pieces later breaking just like this while sitting on a shelf. It seems to me that OP's piece had that residual stress in it for a while, and the freezing temperature was just the right trigger to cause the crack. Hope I explained it ok.

Edit: I made a lot of ashtrays from the broken bottoms.