In sweden it's forbidden to smoke at public transport stops. Nobody cares though so you often have to choose between cancer or getting soaked.
Personally - aside from the smell - I have no concerns about people smoking near me outdoors or even in very well ventilated areas. I understand though that for some people (such as asthmatics) it can be a real problem that goes beyond simply being unpleasant.
Somebody probably drops a spent match, it's still smouldering and it drops inside the escalator where it finds plenty of fuel and begins a fire. From there you mostly just need bad luck - yes the staff could be better trained, but even when they do summon professionals the firefighters don't arrive in time to tackle it while it's still small, the then-unknown trench effect allows the hot gases to pool and initiate flash over suddenly, a bunch of people die.