It’s called an Alarm response.
Basically, one or more of the ants you just blew on has broadcasted a message to all the surrounding ants: “Big thing is taking an interest in us”
This is usually a chemical they put into the air called a pheromone.
They have different pheromones for different threats and also different responses.
For most ant though, “big thing taking interest” would probably be looking to eat a lot of ants. So if the ants were close together, it could just scoop up a lot of ants.
So the ants spread apart to make that harder.
For different threats, the ants might bunch up or even attack as a group.