What causes that, then? I mean I thought it was because it runs rich, but is it a characteristic of standalones or something where it just dumps a bunch of fuel when the throttle closes? Or does it have to do more with the exhaust and how unrestricted it is, or some combo of these things?
Well... it depends. Exhaust temperature and excess fuel can do it. That picture is more than likely due to someone bouncing off of the rev limiter in a boosted car. Boost = extra fuel. Limiter = no spark, meaning more excess fuel going thru the engine. Hot exhaust + open air = explosion.