There isn't any on-car sensor that tells the pump when the tank is full.

The next time you fill up, take a look at the underside of the spout. You'll see
a small hole that connects to a tiny plastic tube that runs up the inside of the
spout to the handle/valve assembly. The normal flow of fuel sucks a small
amount of air up the tube and through the valve mechanism, where it does
nothing. When the tank is getting full, fuel bubbling up from the tank blocks
the hole, creating a small vacuum that triggers the shutoff inside the handle.

Abnormal shutoffs can be cause by a defective nozzle, the little plastic tube
disconnected, dented (metal) or pinched (rubber) fuel inlet pipe to tank,
inlet "flapper" broken off and stuck part way down the inlet pipe, bad
design (I've seen pickups where the gas actually has to "uphill" before
making it to the tank), and assorted other things.

Try pulling the nozzle out a bit, or rotating the handle left or right to get the
fuel to flow smoothly down the pipe. Or just pump slower, but not TOO slow,
or there won't be enough vacuum created to trigger the shutoff, and you 'll be
complaining that you keep spilling gas on your shoes.