As explained so many times, GSX vehicles only see the airport taxiways path structure, they are otherwise "blind" to any of other structures in the airport so, when doing a maneuver that is requited to take a path towards the closes node on the taxiway layout, it's calculated in a way that would minimize the chance to clash into some building.
If the passenger bus just turned around, it would likely clash either in a nearby parking spot, another airplane or possibly a building if it's close enough to the parking spot. Going in reverse is the route with the lowest chance of hitting something.
