Gas bottle stuck to the floor, why did it happen?

When using the fuel, the liquid propane goes through a phase change into a gaseous state. The remaining liquid becomes colder because vaporization takes heat energy. You can see the frost line on the outside of the canister, this is the approximate level of the cooled liquid inside the tank. If temperatures are already close to 0°C the cooling liquid could easily drop it below 0°C freezing any condensation or water between the tank and the floor. If gas is not being used for a while the tank will slowly warm up to the surrounding temperature. It should be safe to pour some warm water on the base as it would take a good bit to heat the propane to a dangerous temperature which would be a lot higher than 0°C.