When you are in a moving bus or car, you are having the inertia of motion associated with the vehicle. So from ground frame of reference, you are moving at the velocity of the vehicle. Now if you suddenly jump out, you’ll fall on the ground with high relative velocity. As soon as your feet touches ground, they will have to come to rest, whereas the upper part of the body tends to move forward owing to its momentum, due to which the person will be disbalanced, fall down and may also skid for some time leading to injuries on the skin. Hence, it is dangerous to jump from a moving vehicle, and higher the speed at which it is moving, it is all the more dangerous.