"Caving" as described above happens for two reasons: 1) the hip size of the kilt is a little too big and 2) the pleats are not pressed properly. The first thing to check is whether the pleats are absolutely the same width from the bottom of the fell to the bottom of the kilt. If they aren't, and they flare toward the bottom, you'll have a wavy bottom edge when you don the kilt. I've posted a particularly egregious example below.

The only solution is to try to re-press the kilt. This can be tough, because, once pressed, it's hard to get the kilt to fold in a new place.

Best to press out the existing creases from both sides using a damp press cloth between the kilt and the iron, and then baste the pleats together at the size you want them. If you skip the basting step, the kilt is likely to fight you when you press, and you'll wind up with the kilt pressed with the pleats in the same place as before.

Barb