Every tuning degrades over time, due to changes in string tension, largely caused by increases and decreases in the wooden soundboard’s crown. Fluctuations in the relative humidity and temperature of the piano’s environment cause the soundboard to absorb and release moisture. The soundboard swells when it absorbs moisture, causing the crown to increase. This increases the down bearing and therefore also the string tension. The opposite happens when the soundboard releases moisture. This fluctuation is unavoidable, but its severity can be managed.
Changes in temperature can also affect the overall pitch of a piano. Substantial temperature changes over a fairly short period of time are bad. Probably the thing that changes the most is the piano wire, because it is far and away the smallest thermal mass. The soundboard would respond to the temperature more slowly, and the rim more slowly still. All are moving, but not at the same rate. All contribute to string tension changes, but the wire contributes the most.
In newer pianos the strings gradually stretch, the soundboard and rim reach a sort of equilibrium with the room’s micro-climate, causing the piano to go flat, while in older pianos the tuning pins can become loose and don’t hold the piano in tune as well. Frequent and hard playing can also cause a piano to go out of tune.