Was it simply stitched afterward? Or did you get the idea from the weaver that it was done on the loom? You can change from stitched double weave to double weave that will open out but it would be easier to just do it afterward. Did the weaving structure appear to change in the pleated area?
Did it look woven all the way or did it look like it was felted in the middle? There are various ways to do this, collapse weave, is the most common. You can also use elastic thread in the center only.