As has been said, (in a roundabout way) it's an excess of back-pressure that's supposed to be caused by the petrol in the tank reaching the tip of the nozzle. It's a safety cutout.

Unfortunately what can happen in practice is the pump feeds petrol in so quickly it doesn't have time to drain the neck of the tank before the neck back fills. It's not uncommon on smaller cars which tend to have a slightly narrower neck on the tank.

The way round it (as has been said) is to not squeeze the grip quite so hard (fnarr-fnarr) so the petrol doesn't flow in quite so vigorously.