While my wife was driving our 2025 Rivian R1S (approx. 13,500 miles), the vehicle was at a complete stop when it began to slowly roll forward on its own, followed by a sudden surge of acceleration, despite no input on the accelerator. This unexpected movement caused the vehicle to collide with a Tesla directly in front and involved another two vehicles. This incident posed a serious safety risk and could have injured pedestrians or other drivers. There were no warning lights or error messages before or during the incident. The cause is UNKNOWN at this time. Rivian has not yet inspected the vehicle, but we are requesting log preservation and a full review. This behavior has not been replicated since, and the vehicle is available for inspection upon request. We believe this may indicate a malfunction in the propulsion system, regenerative braking transition, or creep mode behavior. The incident has been reported to Rivian for investigation.