Answer:
Frieda's unusual behavior is best understood in terms of deindividuation.
Step-by-step explanation:
Deindividuation refers to an individual not seeing him/herself as an individual but rather as part of a group. Since being part of a group creates a new identity for the whole group, it can help the individual loose any self-awareness.
In this case, when Frieda is by herself she is quiet and shy. Once she joins the rock concert crowd she looses her self-awareness and her inhibitions, being able to scream and dance. Once she is part of this group she is in the process of deindividuation.