Usually starts with a little fuel getting into the box. Maybe the rider forgot to shut off the petcock, maybe a sticky float, somehow fuel gets in there.
Then, a twist of the throttle at just the right time while starting the engine leans out the mixture enough (or it is just naturally too lean) to get a carb backfire. This touches off the gas vapors in the airbox and you suddenly get bits of hard black plastic all over the place.
