As the bus accelerates forward, the air inside moves back relative to the bus. The pressure on the back of the bus is higher which will make the balloon floats forward for there is a much lower pressure in front of the bus. Nevertheless, when a short stop happens, the balloon will move backwards due to higher pressure that accumulates in front of the bus. This is an example of general relativity theory where the force you feel when accelerating is indistinguishable from the force of gravity.
