I read a tech article about this maybe 20 years ago. The bottom line was that the filler neck gets too full of fuel and blocks the air being displaced in the tank from escaping the tank until enough air pressure builds up and is enough to push the gas above it out of the way, so it (the air) can escape. This almost always means that gas comes out of the filler neck ahead of the air and onto whomever is unlucky enough to be in the way. Find a way to vent the tank better and it stops happening. A lot of G-Bodies with the smaller unleaded gas openings in the filler neck had this problem. The filler hole for the nozzle for unleaded was smaller so the leaded fuel nozzles wouldn't fit and there was a "trap door" with a little spring across the filler nozzle opening. (Under the gas cap) We used to use our finger to push the "trap door" down into the neck and then bend it out of the way. That almost always stopped the burping fuel.