I've been wearing Buzz's sporrans for 6 years, give or take, and have never had that problem.

As for the "trigger-release" sporran straps, I feel like they could wear the kilt in much the same way as a chain can. I would suggest rigging some kind of stop, that can be attached to the sporran belt, between the two loops on the back of the sporran. Maybe even as little as tightly winding a couple of rubberbands around the sporran belt, such that they are located just to the inside of each of the loops in the back of the sporran. You wouldn't be able to see them, and they might create enough friction to stop the sporran from migrating to one side.

Just a thought.