Fire ant workers swarm defensively prior to a mating flight.
Sexual fire ants typically fly on clear, windless afternoons following a rain. Workers open large holes in the nest, and then spread out, swarming the surrounding area to eliminate any potential threats. Male and female alates are urged out of the nest, where they typically climb surrounding plant material before taking to the air to seek a mate. Below are a few clips of a colony I found swarming on a sidewalk near my house.
See related:
6legs2many 
when you say ‘sexual fire ants,’ do you mean fire ants that are mature and can fly?
That’s right. The wingless workers are sterile.