The fuel comes out of the carb throat. When the air going across the face of the carb from prop wash is moving faster than the air going into and through the carb it will have a siphon effect and suck a bit of fuel out. Another reason they can do it has to do with the pressure drop in the throat of the carb when the throttle is opened, gets a bit technical from there. Many engines do this, velocity stacks will help to minimize it if it can't be tolerated but fuel proofing engine boxes and fuse fronts is a must. Lightening holes in the front bulkhead of a fuse are stupid and should be sealed, especialy down stream of carburetors.