When a body is already moving, the inertia of motion appears to continue in the same state of motion with the same speed and direction until an external, unbalanced force acts on it to change the state of motion.
One's feet instantly come to rest on the ground when one jumps from a moving bus, but the upper part of our body continues to go forward due to inertia of motion.
As a result, when attempting to jump from a moving bus, one may fall down or briefly lose his equilibrium.