請問The door closed behind her, and Mrs. Lippett watched it with dropped jaw, her peroration in mid-air,為什麼closed不是用被動?

有些動詞在同一解釋上，同時可以是及物和不及物(intransitive and stative)，其中相同名詞可以作為動詞的主語或賓語。而"close"是屬於這類動詞,另外例子如"open", "break", "smash"...等等。
~ He closed the door.
~ The door closed.

closed--adjective to the "door"--eg:-The door is closed behind her(=Mrs Lippett)
ie:-When Mrs.Lippett closed the door behind her, she watched (the action-event-scenario-) with dropped jaw, with a closed watch, with a "perforation" of Saliva between mid-air.
ie:-When Mrs.Lippett closed the door behind her, and watched
=active voice,(past tense)
=Not passive voice