A.T. Robertson Commentary Mark 9:38

"John said unto him, Teacher, we saw one casting out demons in thy name; and we forbade him, because he followed not us." — Mark 9:38 (ASV)

Because he followed not us (οτ ουκ ηκολουθε ημιν). Note vivid imperfect tense again. John evidently thought to change the subject from the constraint and embarrassment caused by their dispute. So he told about a case of extra zeal on his part expecting praise from Jesus. Perhaps what Jesus had just said in verse 37 raised a doubt in John's mind as to the propriety of his excessive narrowness. One needs to know the difference between loyalty to Jesus and stickling over one's own narrow prejudices.