Diotrephes is abusing his power and spreading gossip. Gossip reveals passive aggression by isolating and harming the reputation of others, in this case the elder and Gaius. It is passive aggression, because Diotrephes doesn’t attack them actively with accusations. James, the brother of Jesus, warns against the danger of gossip (James 3:1–12).
10 So if I come, I will bring up what he is doing, talking wicked nonsense against us. And not content with that, he refuses to welcome the brothers, and also stops those who want to and puts them out of the church.