A dry atmosphere is the enemy of the natural cork seal. A natural cork is compressed and forced into the bottle as a 100% natural seal. The resilient and elastic cork wants to expand and continuously pushes out against the glass to form a seal.
The air space between the bottom of the cork and the wine is called the ullage. The ullage will increase over time as the cork is far from a perfect seal. It will let some air in while also absorbing some wine, even when the bottles are stored on their sides. Low humidity combined with a defective cork results in the wine moving out of the bottle (increasing ullage) and air naturally moving into the bottle.