The release of anal gas, the culprit having his/her pants (and underwear) far enough down so that the methane gas avoids contact with his/her attire. This method will prevent underwear "stink", and the lack of a barrier between anus and ambience will cause the fragrance to spread quicker and with richer concentration.
Person A: "Did someone shit their pants?"
Person B: (Giddy laughter)
Person A: "What the fuck is that?!"
Person C: "Person B made a bare-ass fart
Person A: "Damn you, Person B."
Person B: "Hey man, I don't want that shit to bake up in my undies."
Person D: "You disgust me."