The pitch of piano strings change for two main reasons; the initial stretching and settling of strings when the piano is new, and movements of the soundboard due to changes in environmental humidity.
When a piano is new, the pitch is seen to drop quickly for the first few years, as the new strings stretch within, and wooden parts settle to the tension. Especially during this period when the piano is still very new, its crucial to maintain optimum string tension, as the piano structure reaches stable equilibrium with the string tension.
Coupled with humidity changes, temperature can also change with the weather, all of which can cause the pitch of a piano to change. The piano’s main acoustical structure, the soundboard is made of wood, which reacts constantly to climate changes.
When relative humidity increases, the soundboard swells, increasing its crowned shape and stretching the piano’s strings to a higher pitch. During drier climatic periods, the soundboard becomes flatter, lowering tension on the strings and causing the pitch to drop.
This constant pitch swing experienced by the piano strings due to its contact with the soundboard, result in a net drop in pitch every year that the piano is not serviced.
Many piano manufacturers recommend 3 to 4 tunings during the first year for a new piano, and at least 2 tunings annually after that.