The phrase "to explain an action when the subject is unknown" suggests a context where the focus is on the action rather than the subject performing it. This aligns with passive voice, which is typically used when the subject of the action is not identified or is deemed less important than the action itself.
In passive voice, the structure often looks like this: "The action was explained (by someone)." The subject (the one performing the action) is either omitted or generalized.
Therefore, the concept you described primarily reflects a passive construction.