The name "Bull Denim" usually refers to a Denim fabric of 100% Cotton in weights heavier than 16oz per fabric yard.

Denim refers to a type of weave where the Warp and Weft yarns are not the same. The warp yarns are plain, un-dyed white, while the Weft yarns are colored.

It is the combination of the Cotton content and the different yarns that cause the problems.
First Cotton will not hold a crease. It's not supposed to. If you are old enough to remember 100% Cotton shirts, they needed Ironing after every washing and the liberal application of starch.
The second is Cotton Will wrinkle. Again, its supposed to.
The third problem with Denim is the different rate the Warp and Weft yarns will shrink. Look at the sides of a pair of Blue Jeans. See the ripples at every stitch line. Those puckers are caused by the difference in shrinkage between the Warp, Weft and stitching thread. Denim will pucker. It will do it at each and every stitch line. It's also shrinking in the middle of the fabric you just don't notice it.

The result is that I have been trying to tell people for years. Denim is a terrible fabric for Kilts. After only three or four washings you end up with a ballet tutu unless you are prepared to spend hours with an iron steaming and stretching back into shape each and every stitch line.

Some of the first Kilts I made were from a 21oz Bull Denim. The fabric was originally used as stage curtains. Heavy and dense this fabric made completely bullet proof Kilts. No swish at all but a wonderful feel when worn. I still have and wear them.
Each pleat edge, inside and out is topstitched. Everything is sealed to prevent anything from fraying. I love them.