You drove and pulled up to the gas station to fill your gas tank. You exited your vehicle, opened your gas tank cover, grabbed and put the dispenser nozzle in. You also had to pay at some point.
How can you forget that the hose is still connected to your car when you drive off?
Your not thinking about it. Muscle memory.
Something distracts you, a phone call, a loud sound, kids, suddenly remembering a urgent matter. You forgot where you were in the process and just skip a step.
Human nature. The same reason drivers can just run into a wall, car and train! Routine becomes ignore and you just forget / zone it out
This is why almost self driving cars are so dangerous. They will throw people into responsible driving mode and they won’t have been paying attention.