Arrears is derived from an old French word that literally just means behind. I've seen it used in English with that meaning in ways that are more generic than specifically referring to payments.
In modern French, it's used both literally and figuratively. A common folk song goes "3 pas en arrière…" (3 steps backward). Similarly, "arriérés" could mean "people with retrograde mindsets", or "arrears", i.e. rent or taxes that should have already been paid. I suppose the English language merged "arriérés" and "arrière" into a single word.
The right expression is la porte arrière (not "en arrière"), which usually means the back door of a vehicle. Some will more correctly say la portière arrière. It will rarely endorse the other meaning of "back door". Out of context, absolutely no one would see anything sexual in this expression.
On the other hand, la porte de derrière will point to the door which is on the back side, for instance the back door of a house. In some contexts, it may convey some sexual innuendo.