The actual gap at the top of the piston stroke is really quite small, it is possible with a badly flooded motor for there to be enough fuel to fill the gap and some.
Fuel as water is not compressable. ie it maintains its volume under pressure. So if you have more fuel than space you have rods bending and cranks failing due to the piston not being able to pass top dead centre causing the motor to stop cold, no slow down so all that momentum in the crank etc becomes destructive force and bang.
Its why motors flooded in fords and floods are in most instances dead, if they were running when the water got them they have rods out the side, snapped cranks. Not good.