I had the same issue, but only when I removed the phone from my holster. I isolated the problem to when the phone came close to the magnet in the flap of the holster. The magnet launched Car Mode and was reproducible every time. My theory is car mounts will be manufactured with a magnet in them so when you mount the phone it automatically launches Car Mode.
I rooted my phone and used ROM Toolbox to freeze Car Mode and now it never happens. Hope that helps.