The phenomenon is called the pilling effect. Its cause is friction caused by some kind of external 'element'. It can be, for example, a staple, a wash label, a velcro strip, the back seam of new jeans, or sometimes even a laser-cut fabric mark on the inside of the pants. Repetitious Rubbing will loosen the fibres with time and it shows as small bumps on the surface of the fabric. Fortunately, 'pilling' does not affect the product's properties in any way, but is more of an aesthetic drawback.