Had a bus transfer @ FRA today. Plane was parked at gate, so busses waited towards the plane's rear. Passengers debarked from front and aft of plane.
Thus, first bus was filled before all J passengers debarking through the front door had reached it.
I really wonder why it is possible in ZRH to have a dedicated J bus, whereas in FRA (and MUC to go slightly off-topic in this thread ) every passenger is treated equally regardless of class of travel booked.