It all comes from how the skirt needs to be grouped in order for it to pick up the leg bends. The hip group has a column of polygons that runs straight down the middle of the skirt, to the ground, Same with left and right thigh. It works okay on non extreme bends (usually under 30°), but it's abilities are exceeded in bends closer to 90° .
Just the limitation of Poser's rigging system.

There's a fair amount of this explained in 'secrets of figure creation for Poser 5' book, by B.L. Render.

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