Why did Johanna hit Katniss in the head?

Why did Johanna hit Katniss in the head? During the escape from the arena at the end of the 75th Hunger Games, Johanna ‘attacked’ Katniss for the purpose of removing a tracking device from her arm, then lead Brutus and Enobaria, the District 2 tributes, away from Katniss.

During the escape from the arena at the end of the 75th Hunger Games, Johanna ‘attacked’ Katniss for the purpose of removing a tracking device from her arm, then lead Brutus and Enobaria, the District 2 tributes, away from Katniss.