Also, those kilts do not have a canvas liner. Without that, the straps are pulling directly on the pleat stitches, which they are not designed to hold. Add a piece of substantial fabric (cotton canvas, washed several times in hot water, works fine) that fastens to the inside of the kilt under the strap attachment locations at both ends, and is loosely attached to the inside of the pleats in between where the straps are sewn down. Sew right through the straps to the canvas This serves as a sort of suspension system, so that the straps are pulling on the canvas instead of the pleats themselves, and the canvas will not stretch. This will stop this problem, and only takes a few minutes to do. The canvas strip only needs to be about 2" wide.
Visualize the head band in a construction hard hat. It grabs your head and the "helmet" just goes along for the ride. That's how the kilt should work.
I've added liners like this to all my "e-kilts" and have had no problems since. I also use them in my X-kilts that are made of lighter fabric (like PV)