This tale is written as survival advice from a narrator claiming to be a U.S. Forest Service search-and-rescue officer. Ranging through deep woods, you may sometimes come upon a staircase standing alone with no context at all — leading nowhere, simply set in the middle of the forest — and the warning is absolute: never approach or climb it, leave at once.
The heart of the horror is an unexplained taboo. Why it is dangerous is never revealed; only the rule "do not" remains. The structure of leaving the unknown sealed and forbidding you to cross its boundary is the essence of rule-based horror, and a variation on the kind of fear where observation or approach itself summons the threat.