Actually, it is a common phenomenon. Mini bikes were originally designed to be ridden by youths. However, as we have aged our gluteus maximus have grown well beyond the intended purpose of the original mini bike seat. When these large posteriors are placed on these tiny seats, a vacuum is created which remains during the length of the ride, during which large amounts of methane gas accumulate. In this vacuum void.the gas becomes highly concentrated. Once the ride is over and the rider dismounts there is a loud suction sound as the atmosphere rushes in and then rapidly expands, carrying with it the VERY odoriferous methane gas. When this gas reaches the nasal passages, repugnance and vomiting have been reported. Hope this answers the question.
FRED