Back-end and front-end are common technical terms nowadays. Traditionally, they are written with a hyphen "back-end". Is there a rule in the English language that dictates this to be a correct way to write term, that signifies general single something? Or did it just "stick" that way?

Clarification 1: The proposed answer that discusses when to hyphenate assumes back-end as a compound word. I don't think it is and this am asking this question. Compound words like on-the-fly are comprised of possible standalone words that are all required to describe the phenomenon or thing in question.

Backend is a synonym to server-side, which really is a compound word. Backend is a singularization of server-side, not a compound itself.

I understand that I may have a logical "gap" somewhere in this line, the reason why I asked is to help me clarify this.

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    There is never almost any logic to whether compound words should be hyphenated or not. – curiousdannii Sep 26 '15 at 11:37
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    Can you please explain why you think it isn't a compound word? – curiousdannii Sep 26 '15 at 11:42
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    Most styleguides recommend that you hyphenate phrases when using them as modifiers ("The back-end subsystem"; "on-the-fly calculation") but not when they are stand alone ("The back end"'; "do it on the fly"). I would not write backend as one word, but I wouldn't be surprised to see it. – Colin Fine Sep 26 '15 at 12:05
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    In general, the more familiar one is with the term the more apt one is to write it as a single compound word. Thus computer geeks will write of "backend optimization", since it's a term they're apt to use quite often (in certain circles). – Hot Licks Sep 26 '15 at 13:04
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    Google ngrams suggest that back-end and backend have increased in popularity recently – Henry Sep 26 '15 at 14:31