Parked 2018 F-150 on slight incline facing downhill. 12' dump trailer attached with load of 2000 lbs. Put transmission in park and set park brake. While preparing to unload trailer, I think I heard a popping sound. Then the truck suddenly began to roll downhill. It traveled about 70 feet before crashing into two parked cars. I ran to the truck and confirmed the transmission was in park and the brake indicator light was on, confirming that I had set the park brake. The truck had rolled freely with no indication of resistance from the brakes or transmission. Considerable damage was done to my truck, the trailer, and the two parked cars. My truck was towed but repairs have not yet begun and no cause has been determined as to why both the park brake and the transmission failed at the same time.