Well, in this case, he wasn't fat. He argued with the flight attendant and cursed, which while the door is still open is easy prey for a "thanks for playing, you're outta here..." action.
Maybe she saw it as him being clear he wasn't interested in "following crewmember instructions",
More likely, his saggy drawers presented a real hazard during an evacuation (how exactly does one manage to exit quickly with a waistband hanging below their hips?....)
Who knows, because we rarely hear the other side of the story.