Have you ever been on an escalator and had the person in front of you just stop at the end instead of going anywhere? Like, just stand there while the escalator propels you into them and the people behind you into you? Stinks, right? Those are the sudden stoppers. In addition to escalators, these people also frequently terrorize elevators, train doors, turnstiles, and revolving doors.
This lack of situational awareness and regard for others adds sudden stoppers to the ever-growing list of The Worst Person Ever.