Bread kept in a refrigerator hardens faster than bread left on the counter, which contradicts the general rule that cold slows spoilage. Staling is a different process from spoilage, and it responds to temperature in the opposite direction.
Staling is recrystallisation, not drying
Baking turns the starch in flour into a disordered, swollen state that holds water and gives fresh crumb its softness. That state is not stable.
As the loaf sits, the starch molecules gradually reassemble into ordered crystalline regions, squeezing out the water they were holding and stiffening the crumb around them.
The bread is not losing moisture to the air so much as redistributing it internally, which is why a stale loaf sealed in a bag is still hard.
The process has a temperature optimum
Recrystallisation needs the molecules to be mobile enough to rearrange but cool enough to settle into an ordered arrangement, and both conditions are met around refrigerator temperature.
Above that range the molecules move too freely to hold an ordered structure, and well below freezing they barely move at all.
The result is a rate curve with a peak near typical fridge settings, so chilling a loaf places it in precisely the worst conditions available.
Freezing stops it almost completely
A frozen loaf passes quickly through the problematic range and then sits at a temperature where rearrangement is effectively halted, which preserves the fresh texture.
Slicing before freezing matters, because a whole loaf spends longer in the intermediate range while thawing, and each pass through that range advances the staling.
Bread taken straight from the freezer to a hot oven or toaster skips the range altogether, which is why frozen bread revives better than refrigerated bread.
Heat reverses part of the change
Warming stale bread supplies enough energy to break up the crystalline regions that have formed, returning some of the softness to the crumb.
The reversal is partial and temporary. Water lost from the loaf entirely cannot be restored, and the bread re-stales faster after each round of reheating.
This is the mechanism behind reviving a day-old loaf in the oven, and behind the disappointment of doing it a second time.
Recipes and ingredients change the rate
Fats, sugars and certain enzymes interfere with the starch reassembling, which is why enriched breads and commercial loaves stay soft longer than a lean flour-and-water bread.
Crust behaves differently again, since it is dry and brittle from the start and absorbs moisture migrating outward from the crumb, softening as the interior firms.
A loaf therefore stales from two directions at once, and storage that suits the crust generally works against the crumb.