Carburetors and older/cheaper fuel injection systems use a mechanical switch to determine how much fuel goes into the engine. This switch is usually controlled by a float that is quite literally floating on some of the gas that is entering the engine. If too much gas is entering the engine, the float rises and pushes the switch closed. When the level of gasoline drops, the float sinks to allow more in.

The buoyancy of that switch is calibrated against the amount of gas that the fuel pump is delivering to the engine - which will be a fixed value. IE, if the fuel pump is delivering 1 psi of fuel to the engine, the float will be set to press on the switch with 1 psi of pressure when the fuel supply needs to be cut off.

Pumping fuel into your car can increase the amount of pressure in the system, which can increase the amount of pressure coming out of the fuel pump - particularly when the tank is near full.

So imagine that your tank filling up causes a pressure surge and that 1 psi of fuel normally coming out of the fuel pump briefly spikes to 5 psi. That's going to mean that the switch in your carburetor/fuel injector doesn't close and more fuel keeps going in even though the engine is full. That could potentially flood your engine.

A flooded engine can be anything from a minor inconvenience to something that burns out your starter motor.