It may cause fire

There’s a chance where the outflowing fuel may catch fire in the fuel tank and burst into flames. Therefore it is not advisable to pump petrol without turning off the car engine. Check out the video in https://www.motorist.sg/article/163/this-is-what-happens-when-you-pump-petrol-with-the-car-engine-still-running to see why you should always switch off your car engine while pumping petrol!