What do you call it when you come up with a phrase to fit an acronym?
This is called a backronym or bacronym:
A backronym, or bacronym, is a constructed phrase that purports to be the source of a word that is an acronym.
(source: Wikipedia)
They give the AMBER Alert as an example; it was named after a girl whose name was Amber, but constructed the phrase "America's Missing: Broadcast Emergency Response" to fit the abbreviation.