This just happened to me today .
My driveway is on a slop and I put it in park , set the parking brake and started to get out of the truck as it started to roll back . Only way I could get it to stay put was to put it in 4 high . It has done this twice in the last 5 years but is random when it does this . Drove it later that day and had no issues with it rolling down my driveway.