Probably a bad alloy mix or something. I rode my 12 year old cruiser in the polar vortex, and it too was completely fine. I would grab the serial number & contact manufacturer. Alu does not, in my experience, just crack. Something must be wrong with the alloy mixture or was not aneeled properly or something. Even expanding corrosion shouldn't do that to one of the thickest pieces on the frame.

My old cruiser did have one crack in it, however that was definitely from crashes, and NOT from cold.

- Andy