Bottle top that flew off

 

We were sat at table when the blue top just flew off. It was a very hot day. My first thought was that the milk had gone off and a build up of gas from the bacteria had increased the pressure enough to blow off the lid. But the bottle had not long come out of the fridge and was cold. So my next thought was that the cold air in the bottle had lower pressure than the warm air in the room. As thermal energy conducted into the air in the bottle, the extra energy made the air particles move faster, increasing the pressure until it was greater than atmospheric and the lid blew off. But now that makes no sense because the temperature in the air in the bottle cannot be greater than that in the air. Maybe the colder air, being more compact, had more particles in the same volume which could lead to a higher pressure than the external when thermal equilibrium was reached.