I've done some experiments, after reading some of The Wizard of BC's advice on this issue, and this could be in the steeking of the kilt just above the bottom of the fell on the inside of the kilt.

The pleats on the left side want to sag outward with the fell stitching being a pivit point at the bottom. The steeking holds them in place on the other edge of the pleats.

It might be a matter of adjusting the steeking so the pleat is held strait. I don't know if your pleats are cut away above the fell on the inside, though.

* Sorry, it took a bit to write that. Someone else should try to explain it better.