Are you sure it's aggression and not just rough play? Were they making any vocal sounds while they did this?

In my experience, serious fights are always accompanied by lots of vocal yowling while play is usually quiet.

I have also seen cases where the cats are unevenly matched in size; they want to play but the play becomes too rough for the little one, so the little one protests with some little whimpers, which usually makes the big one back off a bit.