It's hard to tell; mating and fighting can look pretty similar!
However, it's more than likely just a dominance thing; shoaling fish can form quite complex hierarchies. You do need to make sure you have enough fish, of the same species, to form a proper shoal; that's at least six, but preferably eight or more. That way one fish won't be able to pick on one or two others, and any aggression is spread more evenly.
You don't need to worry about reproduction; most tetras are very difficult to breed in a community tank, as they and any other fish will eat any eggs before they have a chance to hatch.