‘And John answered and said, “Master, we saw one casting out demons in your name, and we forbade him, because he does not follow with us.” '
As mentioned the disciples had seen a man casting out evil spirits in the name of Jesus even though he was not one who outwardly followed Jesus. And so they took it on themselves to forbid him, without consulting Jesus. No doubt they had thought, ‘How dare he use the name of Jesus like this. We are the only ones who are allowed to use the name of Jesus!'