Rankins Dragons - bullying or mating?

I have 3 rankins dragons in the same (4 ft viv). Rankins are supposed to like living in communities unlike beardies. They are now almost a year old and I was told (when I got them at 4 months) they were likely all females ("Reptile Guy" looked and poked about a bit).

For the first time I have noticed this evening when I put out the basking light (leaving on the UV) that one of the larger dragons jumped on the smallest one's back and started biting its neck. I immediately panicked and took out the smaller one not being sure if it was a mating thing (they are related so is prob a good idea not to let them mate) or if it's bullying. How does one tell? The smaller one is about 55 grams, the one that jumped on her/him is about 60 grams (the largest is 75 grams) - these aren't beardies but rankins so are smaller.

I'm unsure now what I should do - separate the "little one" - he looks smaller (but not significantly so)- I'd have to get another (small) viv or let them be and keep a close idea - it's the first time I've ever noticed this happening but will keep an eye on it in future. I have them all out together during the day sometimes and they sit on the windowsill together. Should there be any other signs I should be looking for to give me a clue as to which is happening - the bullying or the mating?

I still have the "little one" out at the moment and thougt I'd keep her out until the others have gone to sleep.