I've got six black skirt tetras - three females and three males. Right now, all three males and one female are swimming very quickly up and down the tank. The males look like they're rubbing against the females sides when they can catch her, otherwise they're following as close as they can get.

Usually all six swim much slower and in a looser group. The other two females are acting normally, but aren't being chased like the biggest one is.

Fairly typical tetra behaviour, males constantly drive females.