the petrol shut off because your tank was near to filling up, i asked a similar question on here once and i was told that there is a small tube at the end of the pump, now obviously when you pour the petrol your tank will fill at somepoint, and what happens is when the pump is in, if it gets covered in petrol (indicating that your tank is full) it means that the small tube at the end gets filled with petrol instead of air, the pump recognises this and shuts off the petrol supply as a safey feature. but im not sure how you can then keep pumping petrol anyway because surely you could just keep going until petrol came pouring out!