Was it already frozen when you transferred it to the fridge? That kind of sounds like it was supercooled and it happened to crash out while in the fridge. Once

Was it already frozen when you transferred it to the fridge? That kind of sounds like it was supercooled and it happened to crash out while in the fridge. Once it freezes that's as high as the pressure will ever get, thawing it will only allow more CO2 to redissolve and lower the pressure.