Females are often aggressive over the males, so I would not be concerned in regards to this being an abnormality. However, with this being said, monitor her behaviour. If dominance persists over the two, they should be separated immediately, as they have the potential to severely injure each other.
As for a possibility as to why this occurred, it is possible to be something as simple as the fact that she saw a dragon in her sight, and became aggressive protecting her territory. I do not particularly condone housing two dragons together permanently, however it is your choice and if you continue to do so, please be sure to monitor their behaviour.
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