2006 GMC Sierra gear changer

Hello,
I have a GMC Sierra 2006. I was in my driveway. I got in and started the truck, then I realized that my gate was unlocked, so I got out to go and lock it. I opened the driver door and got out. I had walked about 5 steps away and then I realized that the truck was rolling backwards in my driveway. I didn't think that once you took your foot off of the brakes, it was possible for the truck to move. For example how could I have gotten out of the car, walked away, then the car stays stationery, then out of the blue, the car reverses? It was very bizarre. This seems like a gear malfunction. Have you heard of this defect on the GMC 2006 Sierra? Please advise! Thank you.