Several explanations are put forward for these heightened "coifs":
(1) Competition among local women; when one saw a bonnet higher than hers, she added a centimeter, and so on.
(2) A bottle could be hidden in it.
(3) Louis IV knocked down Breton church steeples to quelch rebellion, so women made their bonnets higher to rebel against this; we later learned the dates don't bear this out.