Sorry dehammer, but that doesn't cut it. First off, I didn't say it froze when I opened it, just when i took it out. And there is no way the water bottle is "under pressure". The tiny amount of air in the top of the bottle is lower pressure relative to the pressure at room temp because pV=nRT.
Also your understanding of pressure is a little off. Cold air is not heavier, it's denser. There is no way you can convince a chemist or physicist that the pressure in a freezer is higher than the pressure outside, all other things constant. Please referrence ideal gas law above. And, pressure is force per unit area. Yes, cold air is denser but not heavier. So consider one molecule hitting the surface of a container and another molecule of the same mass hitting the same container at a higher speed. Since momentum = mass * velocity and mass is the same, the warmer molecule has greater momentum since its travelling faster.
I'm not sure I follow your post the way you meant it, but you should also know that water, as with most liquids, is almost totally incompressible. You can't simply change its pressure by changing the pressure of its environment.
