changing bus drivers

Sometimes, when I take a later bus to work, the drivers change at Rosengarten Strasse. Today was one of those days. I was sitting towards the front of the bus so I heard the exchange. When the new bus driver boarded he said "Gruetzi" to everyone and everyone responded "Gruetzi". It was like school children say good morning to their teacher as she enters the room and starts the lesson. I thought it was amusing.