Rents go down under pretty much one circumstance: demand drops. People leave an area, or local economics suffer so people double up in housing, or people die.

Rents go down under pretty much one circumstance: demand drops. People leave an area, or local economics suffer so people double up in housing, or people die. If there are any examples of times/places where rents have fallen in nominal or real terms, they will almost all coincide with this kind of demand drop. Supply-related rent drops are theoretical and almost never happen in real life. Supply is important anyway, because otherwise rent growth accelerates faster, but the market will almost never supply housing to the point where rent change is negative.