I have the exact same issue. Worn tires moved up front, treaded tires out back. Totally getting DSC in turns. Going today to get new tires. In my case the fronts are down to the wear bars anyways.
1. The worn tires and good tires have different diameters throwing off the system.
2. As the previous poster noted, the worn tires have more (or less) grip and causes imbalance.
I did not have the issue before rotation.