back early on in this trucks development mcfa came out with a transmission update that required the installation of a wireharness and software update that requred the truck to sense movement if in neutral or off the operator seat. It was designed to sense the unintended movement of the truck in such cases and force the truck to shut down for safety purposes to prevent the unintentional rolloff scenario that could be caused if the truck was left running unattended without the parking brake set holding the truck in place.

this is a normal operation of the truck
The code F99 is displayed on the dash if the DCM observes this scenario and shuts the truck down. The Key will have to be recycled to restart the truck.