Fire ants workers release an alarm pheromone if they recognize another ant as a non-nestmate or if they encounter some other disturbance. When the alarm pheromone is detected by a nestmate, the ant moves rapidly in what appears to be an erratic pattern toward the alarm pheromone source until it encounters another stimulus. Then it may release an alarm pheromone, which causes the other ants in the colony to bite and sting.
Fire ants may crawl up the leg of a person who is unknowingly standing where the ants are active. When the person begins scratching or moving, the activity induces one or more ants to release an alarm pheromone and sting. The alarm pheromone is very volatile and acts quickly on the nearby ants, which then begin to sting. It seems as though they are stinging all at the same time.
Find more information about fire ants in eXtension's Imported Fire Ant Resource Area.