This is a common bug when they use the /back command, it basically means he is using the bring command to tp you, or he has phys-gunned you up, then tried to teleport you back after the sit is over, with no location for you to go back too.
He SHOULD have been taught in training not to do this, but these days it feels like training is a pre-school class session, with no one caring or paying attention to the important things.