I literally just breed peach faces and have 2 little babies. I'm no expert but can explain how things happened for me:
When I got my pair I didn't even know if they were male or female but when one started nesting and the other one didn't even know how to I got an inclination they were male and female. As the female started building the nest she got very territorial and very nippy to the male. Until things were prepared and only when she was ready did I see them mating. The rest is history.
Does the aggression draw blood? If not, I would, from my experience, say it's breeding behaviour