We have several horses (minis) here with lacing, so a few years ago I did some popping around on the internet searching for information.
The only thing I found was that it was felt it was genetic and they were working on the theory that it was from the Sabino gene. At that point tests hadn't been completed to be conclusive.
For me, it made sense. All of the horses we have with lacing are related -- at least distantly, and there was pinto involved in the bloodline.