True that we do not know the direction of effects. Here, we use predictor in the statistical sense not causal sense.
First, some factors (i.e., CU traits) may be predictors whereas others (i.e., delinquency) may be correlates or consequences of relational aggression. If the authors are really interested in the processes that could feed into reactive or proactive relational aggression in females, they should have been more attentive to this issue.