Carrier/rack problems of the 2019 Chevrolet Silverado 1500

I was driving on the highway for approximately 1 hour/75 miles. I pulled over to clean out my truck. I parked in a vacant parking lot. The truck's engine was running, the vehicle was in park. It was parked at the top of a hill. I got out of the drivers compartment, and walked around behind to the passenger's side, and cleaned out some trash. I then walked back around to the rear drivers side door, and removed more trash. I walked about 15-25 feet away, to put the trash in the garbage can. The truck made a weird �chirp-chirp-chirp� alarm/electrical type noise, and then drove backwards. The reverse lights were on. I ran after the truck, the drivers door was locked. I pushed the unlock button on the door and hopped up on the running board. I opened the drivers door and jumped in, and slammed on the brakes. I stopped the truck, literally one foot from going over an 8 foot concrete embankment. I am really terrified by what happened. It was in park. It has that new system where the keys stay in your pocket and everything is push button (start, unlock) the vehicle's column shifter was still in park when I stopped the truck. Photos: 1. ) this is the hill, and space I was parked in. 2. ) these are my tire tracks, my trucks tires hit that sheet metal, and you can see the approximate 8 foot drop with the dumpsters. 3. ) this shows the concrete speed bump thing that my truck backed over when it was rolling out of control this should not have been mechanically possible. If it was in park it can't happen (theoretically) if it was in reverse, it would have backed up immediately after letting my foot off the brake. If it had been in neutral, it would have drifted backwards immediately due to the grade of the hill. If it had been in drive or low, it would have moved forward, or stayed still.