Fun times in the bus, episode #2:

A lady is standing, facing the back of the bus. The bus is stopped at a red light. She's not holding on to the pole.

The light turns green and the bus starts going forward.

Said lady loses her balance and falls back. She extends her hand to her side, hoping to grab anything to help steady herself again.

The "anything" her hand encounters would be the butt of the man standing behind her, facing the front of the bus.