When you are in bus, you are moving with the velocity of bus with respect to ground. So you have the interia which can be understood as your momentum in this case. So when u jump out of a bus, you change your speed from a finite value to zero instantaneously, which causes an abrupt change in momentum, which your body will try to resist. So your upper body will still be in motion while your feet are still. It means you will definitely fall down.