One reason for piano to go out of tune is rapid change of relative humidity. Wood expands when absorbing water droplets; remember those warped wood pieces of bookshelves? Piano soundboard are made of thin wood pieces, which resonates with strings. The environment can be humid, but if the humidity changes rapidly, the string tension go up an down… making the piano out of tune.

I log the relative humidity of my room. Even with windows closed all the time, the humidity keeps changing. It rained yesterday, the meter went up to about 85%. This morning, it dropped 20%. This is ok. But with air conditioner, the humidity would go to 4x%, creating a 40% difference. In places where it snows, the heating system might result in 20% or below. Piano soundboard might crack.

Of course, every piano react differently to humidity; such as, how frequent and rigorous it is used on a daily basis. But if the piano goes out of tune quickly, it might worth investigating potential causes, including humidity changes.