[mleonhard]

This happens when the air in the bottle warms up and increases pressure which tries to expand the bottle. To prevent this, fill the bottle with hot water, close it, shake it for a moment to let the air mix with the hot water and warm up, then open it just enough to equalize the air pressure, and close it tightly. As the bottle cools, its internal pressure goes down. External air pressure will try to crush the bottle. External pressure is less likely to damage the bottle than internal pressure.