* Temperature and humidity changes. Violins are made of wood, which is a hygroscopic material that absorbs and releases moisture from the air. As the temperature and humidity change, the wood in the violin will expand or contract, causing the strings to change in tension and go out of tune.
* String stretching. When you play the violin, the strings are stretched and pulled as you press down on them with your fingers. Over time, this can cause the strings to stretch permanently, which will put them out of tune.
* Wear and tear. As you play the violin, the strings will eventually wear out and need to be replaced. Old strings can become stretched, loose, or even break, which will all cause the violin to go out of tune.
* Bridge or soundpost adjustment. The bridge and soundpost are two important parts of the violin that hold the strings in place and transfer their vibrations to the body of the instrument. If either of these parts is out of adjustment, it can cause the strings to go out of tune.
* Peg slippage. The pegs are the small wooden pegs that hold the strings in place at the top of the violin. If the pegs are not tight enough, they can slip, which will cause the strings to go out of tune.
It is important to keep your violin in tune so that it sounds its best. You can do this by periodically checking the tuning and making adjustments as needed. If you are not comfortable making adjustments yourself, you can take your violin to a qualified violin technician for service.