It’s what happens to phrasal verbs when they become nouns and adjectives. I couldn’t find a great source other than this writing tip, but “back up” altogether makes up a phrasal verb.
English is rather notorious for having these phrases that when translated directly don’t make any sense: the words on their own are rather meaningless and have to be taken together to derive the meaning:
- speak up (verb)
- slow down (verb) → slowdown (noun)
- shut down (verb) → shutdown (noun)
- piss off
For example, I can’t translate “speak up” to “hablar arriba” in Spanish, that makes absolutely no sense.