Does anybody have any idea why "doors" in the idiom "behind closed doors" is "in the plural"? I have seen "behind bars" and the plural form "bars" makes sense, but I wonder why it should be "doors" not "door". Maybe the answer lies in the origin of this idiom but I couldn't find anything on the Internet.
I do not think it refers to the number of panels on the entryway, but rather to the number of entryways one would have to pass to get to the event.
It might come from the tradition of having a room reachable only from the estate owner's private bedroom - a cabinet. That is, there are multiple doors closed between the event and the outside. So the event is so private that it is being held in a far interior room.
A lot of idioms have no rhyme or reason to them. They are that way because that's how they've always been said.
If you're looking for logical explanations, then the visual is that there is more than one door between you and whatever is going on behind the last door -- a secret meeting, an illicit affair, a clandestine encounter, etc.