What makes my piano go out of tune?

Simply stated, Humidity or the lack of it.
The soundboard on the back of your piano(it will look like boards laid diagonally 45deg.) either absorbs or dissipates water(humidity). This changes the dimensions of the soundboard and either stretches or loosens the strings knocking them out of tune. When we are talking about tuning though we are talking about minute levels of movement to make a piano sound horrible. Some piano's will go out of tune all together some will not. The latter of these is very noticeable it will start to have a ragged sound to it.
Moore's Piano Service