Why do broken escalators throw us off balance?

From the Atlantic:

The escalators on the D.C. Metro are very tall and they are always breaking. This sucks a little if you are just lazy like me, it sucks a lot if you’re on crutches or you have a suitcase. But even when you know the escalator is broken, and expect it not to be moving, for many people, there’s still a moment of disorientation when you step on and start climbing, a weird imbalance that doesn’t happen with equally motionless stairs. But a broken escalator is stairs. Hence the mystery.

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Why do broken escalators throw us off balance?