Somewhere in some office of some person who has never driven a bus except in training, they have decided that the best way to create greater guest interaction is to have the driver stand outside the bus while the guests are boarding.
The reality is most guest do not feel the bus expererience is very magical and so they are throwing idea darts in the dark trying to hit the "magic" bullseye.
The concentration shouldn't be on drivers sitting in a seat or standing outside so much as the spiels delivered over the PA systems, the packing of busses with 80 guests to the point people are on top of each other, and general personality issues with some of the drivers. Standing or sitting a grumpy is still a grumpy.
The number one focus at Disney by their own words is safety but in this case they have forgotten that having a driver outside the bus creates a potentially unsafe situation. As I stated previously, these types of accidents have happened before but with the new policy the frequency will increase.
I would imagine in the case of the pictures above the driver received full blame for the accident as he or she did fail to set the brake. The contributing factor will be ignored. In all places that I have worked as a driver and in management, if the bus is running, the driver should be in the bus. Otherwise the bus should not be running.
We wont even talk about the travel time increases of the drivers getting on and off the bus all the time. SO much for great service.