Different fabrics, different rates
A cotton or linen face fabric may shrink several percent in a warm wash. A polyester lining shrinks almost none. Sewn together along their edges, the two now fight each other: the face pulls up, the lining hangs long, and the panel puckers along the seams and sits crooked at the hem. Even a perfectly even shrink is a problem, because a floor-length drape that loses two percent of its length no longer reaches the floor and cannot be let down.