This problem is more common on softer woods like maple, rosewood, cocobolo or boxwood.
It occurs when the instrument is stored incorrectly. After it has been played, the humidity will increase, and then the instrument will dry out again. During that time the wood will shrink, but depending on the way the instrument is stored determines if it will keep its shape. Our advice is to store it standing straight up or upside down. This way the water can run out and the body will keep its shape.