Why do some clothes shrink when washed?

Fibrous materials shrink when they come in contact with water. Their subsequent shrinkage can be ascribed to swelling and release of internal strains. Release of strains imposed previously takes place when the material is wet. 

Swelling that occurs when water is absorbed produces internal rearrangement that can result in external shrinkage. Whereas fibres and yarns undergo shrinkage to a small extent when wetted with water, considerable shrinkage can take place when a fabric is wetted with water due to the peculiar structure of the fabrics. 

In the fabric, lengthwise thread is called warp and breadth wise thread is called weft. The difference between the thread length and the cloth length is the crimp of the thread. If the cloth is wetted, the diameter of the threads increases and if the crossing weft threads were to remain the same distance apart from one another the path of the warp thread would be longer, that is, it would be stretched. If the weft threads move nearer to each other then the path of the warp is shortened so that it can span the distance without stretching or with much less stretching. If the cloth is not constrained at the ends of its length, there is nothing to stop the weft threads from moving nearer to each other and the cloth shrinks warp-wise. Thus the cloth must shrink one way or other or both when washed.

Source: thehindu.com