Zavim, Chapter Four, Mishnah Three
1) If he knocked against a door, doorbolt, lock, oar, mill basket, or against a weak tree, or weak branch of a strong tree, or against an Egyptian ladder unsecured by nails, or against a bridge, beam or door, not made secure with clay, they become unclean.
2) [If he knocked] against a chest, box or cupboard, they become unclean.
a) Rabbi Nehemiah and Rabbi Shimon declare them clean in these cases.
Section one: In all of these cases the zav knocks against something and thereby causes an object to fall off of it. Since these things are loose or weak, we can assume that the zav caused the object to fall and therefore the object becomes unclean. Many of these items were mentioned in mishnah 3:1 (and their opposites in 3:3).
[Note that door seems to have crept into this mishnah twice. Clearly a mistakeit probably belongs in the first half and not the second].
Section two: According to the first opinion these objects are not so sturdy and therefore it is likely that the zav caused them to fall. Rabbi Nehemiah and Rabbi Shimon believe that they are sturdy and therefore if something falls off of them, it probably was not due to the zav.