The contact owns a 2021 Kia K5. The contact stated while her daughter was driving 60 MPH, there was an abnormal sound coming from the engine, when the gas tank expanded, and the back seat popped up. The contact stated that two teenagers were in the back seat, and one was injured. The contact's daughter pulled over and waited for a short while. The vehicle was restarted; however, the vehicle drove slowly. The vehicle was taken to a dealer; however, the vehicle was not diagnosed. The vehicle was taken to another dealer; however, no cause for the failure was found. The contact was informed that the seat had moved two inches. The contact stated that the vehicle was not repaired because the frame of the seats could be damaged. The vehicle was not repaired. The manufacturer was notified of the failure. The vehicle remained at the dealer and had not been driven since. The failure mileage was approximately 86,000.