I’m omitting the phrase “It’s actually incorrect because” from this discussion since it’s not relevant.

“[T]hey would probably be fired at from their behind”

“be fired at” is a transitive verb phrase, so there are necessarily two related entities in this scenario: the firer and the target, the latter of which we know is “they”. Simply saying “be fired at from behind” could indicate that the firer is aiming backwards as a trick shot since it is ambiguous if “behind” refers to that of the target or if it instead refers to the act of firing; most people could figure out the meaning in context, but I prefer to avoid asking that from my audience. Since “be fired at from their behind” uses “their” as an appositive to refer to the entity or entities yet mentioned in the sentence, and the only one yet mentioned was the target (“they”), this clarifies that “behind” must refer to that of the target.

It’s also commonly, but not absolutely, considered grammatically incorrect to end a sentence with a preposition. “from behind” ends with a preposition, whereas “from their behind” ends with a noun (“behind” not meaning their buttocks but rather that which is located to their rear).

I was trying to get the comment posted before a particular time constraint, so I didn’t have time to locate a word that singularly refers to the area located to one’s rear.

You’re welcome to consult !english@lemmy.ca about this.